Congruent is a new term for an idea we have already been using. We say that two figures are congruent if one can be lined up exactly with the other by a sequence of rigid transformations. For example, triangle $EFD$ is congruent to triangle $ABC$ because they can be matched up by reflecting triangle $ABC$ across $AC$ followed by the translation shown by the arrow. Notice that all corresponding angles and side lengths are equal.
Here are some other facts about congruent figures:
We don’t need to check all the measurements to prove two figures are congruent; we just have to find a sequence of rigid transformations that match up the figures.
A figure that looks like a mirror image of another figure can be congruent to it. This means there must be a reflection in the sequence of transformations that matches up the figures.
Since two congruent polygons have the same area and the same perimeter, one way to show that two polygons are not congruent is to show that they have a different perimeter or area.