Two line segments are congruent when they are equal in length.
For Example;
In the above image, both the line segment are congruent since they have equal length.
AB = 4 cm
CD = 4 cm
For congruency, the lines can be at different angles with each other.
Hence, there is no rule for any angle formation between the congruent lines.
Symbol for congruent lines
In geometry congruency between lines is shown by symbol \mathtt{\cong } .
Hence, if lines AB & CD are congruent to each other, we can write;
AB \mathtt{\cong } CD
Try to practice these symbols as they are directly asked in geometry exams.
Conclusion
When line segments are said to be congruent it means that they are equal in length.