It is a function which can be evaluated at compile time, conceptually similar to constexpr functions in C++. That is currently a nightly-only feature, although work has begun on stabilizing calls to const fns. The name comes from the fact that you can assign the result of a const fn to a const.
There are two broad use cases. One is to make sure that the compiler can pre-compute some quantities at compilation time when that is necessary for good run-time performance. The other is to derive quantities which have to be known at compile time, such as the length of arrays.