A “library” is basically just some sort of collection of molecules. Often this is a large collection that is worked with in a high-throughout manner (think robotic liquid-handling machines and stuff). There are tons of different kinds of libraries that can be classified by what they contain and/or what they’re used for. Two of the main types are sequencing libraries, where your goal is to characterize what’s in the library; and screening libraries where the library is a tool for you to find molecules that bind a target, have a certain effect on cells, etc. (compound libraries, fragment libraries, etc.) or genes that play a role in some process (CRISPR guide libraries, siRNA libraries, etc.).
