Two objects are congruent if they have the same dimensions and shape. Very loosely, you can think of it as meaning 'equal', but it has a very precise meaning that you should understand completely, especially for complex shapes such as polygons.
Congruent polygons have an equal number of sides, and all the corresponding sides and angles are congruent. However, they can be in a different location, rotated or flipped over. So for example the two triangles shown above are congruent even though one is a mirror image of the other. See Congruent Polygons
Congruent polygons should not be confused with similar polygons which can be in the same proportions but different sizes.
The symbol for congruence is
If A and B are two objects being compared, such as line segments, angles, triangles etc, then the statement is read as "A is congruent to B".
If there is a diagonal line through the symbol, this means 'not': is read as "A is not congruent to B".