The term "Magicule" is derived from "Magic" and "Molecule, though it should be noted that the term's Kanji itself are actually referring to a magical/demonic Chemical Element or Atom instead, so describing it as a Molecule might be considered a mistranslation by the official translators. In addition, the Kanji being translated as "Magical" could alternatively be interpreted as "Demonic" as well, which is why Magicules are usually associated with Monsters, who are creatures who are innately based on Magicules and whose bodies tend to be more based on Magicules than physical matter the higher their Rank is.
Although a special kind of energy, it is still able to have causal effects on regular physical matter and physical phenomena as well. Many Spiritual Lifeforms are entirely made up of Magicules and thus physical attacks do not have any proper effect on them. One would need magical attacks, or Skills, or Arts, or weapons that can affect non-physical matter to cause proper damage, an example of which would be when Elen was firing her Icicle Lance Magic at Ifrit.
Magicules are analogous to "Mana," or "Magic Points," or "MP" in games and other shows. Magicules, however, are more accurately "Magic Power" rather than "Magic Point," since even if the World is game-like, it isn't an actual game.
The exact natural laws that govern magicules in relation to physics are not known but there are clear deviations from the usual physical laws like the laws of Thermodynamics or Energy Conservation being outright broken. However, Magicules are capable of reproducing physical phenomena via the "Manipulation" Extra Skills like Water Manipulation, Fire Manipulation, or Molecule Manipulation, which work via Magicules interacting with physical matter to produce the desired physical phenomena.
Effects on Organic Lifeforms
To any lifeforms that are not Monsters or Humans that turned themselves into Majin (Humans usually possess no or only very little amounts of Magicules in their bodies), Magicules are toxic and might even cause them to die if the Magicule concentration is high enough.
However, there are exceptions like some animals, plants, or Humans that might survive the toxic effects of the demonic matter and instead mutate to become Monsters themselves. Other exceptions are those who build up innate resistance against Magicules or have obtained anti-Magicule physiques like Sages and Saints. Other than that, even Spirits are susceptible to Magicules, since coming into contact with Magicules might corrupt them and turn them into half-monsters, half-spirits. Even the Queen of Spirits is no exception to that.
It should be noted that even Monsters themselves got a tolerance limit, however, that has only been shown by those with Rank S or above so far. Veldora's aura (Rank Special S), when sealed in the Sealed Cave, prevented any Monsters below Rank A from approaching him so only Rimuru who was Rank A upon birth was able to do so without being hurt. However, later Rimuru after exiting the cave, and unconsciously releasing his aura, did not hurt the Goblins with it despite Rimuru being either Rank A-plus or Special A at the time, while the Goblins were at merely at Rank E. However, later during the Falmas Invasion arc, Rimuru, who even before his ascension reached Rank S according to Razen, was unconsciously releasing his aura in anger upon learning of the early attack by the Falmas Otherworlders, causing the weaker monsters around him to actually be somewhat hurt by its effects, however, it should be noted that at the time those monsters were also weakened by the Prison Field Barrier, while Rimuru himself was not affected by the barrier due to his own Multilayer Barrier canceling its effects, the lower range of Rank S might normally not be enough.
There is also the Skill "Demon Lord's Haki" that mixes the user's aura with the user's Magicules making the effects of the user's aura more potent, to the point of being able to use the aura as a weapon, as it's made from the combination of the Skills "Coercion" and "Magic Aura."
Although Magicules are described as the building blocks of (magical/demonic) life, they are also capable of affecting inanimate objects as well. In most cases, only in regions with high Magicule concentrations, such as the Sealed Cave and Rimuru's Stomach, inorganic and inanimate substances can absorb Magicules to become an organic substance, such as Magic Steel. In such cases, the substance remains an inanimate object but obtains features similar to that of organic matter, such as reshaping according to the user's will, etc.
In the World, Yuuki Kagurazaka created a system of ranking monsters according to how much Magicules they possess and how intelligent they are. The higher their Magicules, the more dangerous the creature is, and thus the higher the ranking.