As a verb: It means to bring something into existence by magic or some other supernatural power, or to call upon or evoke something (like a memory or a feeling).
As a noun: It's a magical or supernatural act or spell, usually involving the summoning of spirits or other supernatural beings.
The specific part of speech depends on how it's used in a sentence.
Examples:
* Verb: "The magician conjured a rabbit out of a hat."
* Noun: "The ancient conjures were said to be powerful."