A polyhedron is said to be a truncated polyhedron when you cut across its original vertices to develop a new facet in place of each vertex or polytope vertex to be more specific. So in simple words, truncation is the process of cutting at vertices to produce new facets on the same polygon.

Depending on the depth of cut, where and how it is applied there are different types of truncation that can be applied on a polygon, Ex: Uniform, Shallow, Edge, Partial, etc.

You can refer this page on wikipedia for more info - https://en.wikipedia.org/wiki/Truncation_(geometry)