What is the Nix store?

The Nix store is a folder, located at /nix/store by default, that contains every build result from a derivation that Nix has ever generated. These build results stay in the Nix store until they are explicitly garbage-collected. Each entry in the Nix store is prefixed by a hash of the derivation that was used to build - this is how Nix avoids building the same thing more than once, and how it ensures that there is a unique reference to every possible version and variant of a piece of software.

You should not ever need to touch the Nix store manually; it is entirely under the control of Nix. However, Nix does provide several utilities for managing the store, such as nix-collect-garbage.