const generics, also known as generic constants, are constant values that are parameterized into the type of a value. For example, a u64 array of length 10 has a type of [u64; 10]. The length, 10, is part of the type signature.
For a long time, arrays had a special place in the type system for their length. This quirky feature of arrays has since been extended to any type definition that could benefit from constant parameters.
To define a generic constant, the constant parameters are placed at the end of the parameterized types with a special const keyword to mark them as const generics. The most immediate application of this new feature is to enable more meaningful interaction with arrays of generic size.