The document is from a geometry textbook and covers congruent triangles. It defines congruent triangles as those that are the same size and shape, with corresponding angles and sides being equal. Examples are provided of naming corresponding parts of congruent triangles and using properties of congruent triangles to prove other triangles are congruent or find missing angle measures. An engineering application example proves two triangles congruent based on bisecting lines and equal corresponding parts.