Question

What does AB(line over AB) is congruent to CD(line over CD) mean? How is it different than AB= CD?

Answer 1

$\overline{AB}$ denotes a segment $AB$, that is the set of points $A,B$ and all the points between them. The distance $AB$ is called the length of $\overline{AB}$. Two segments are said to be congruent if they have equal lengths.

Therefore, $\overline{AB}$ is congruent to $\overline{CD}$ means that the length of $\overline{AB}$ is equal to the length of $\overline{CD}$. This is the same that $AB=CD.$ Therefore, there is no difference between these statements.