The line is a basic geometry concept.
You can think of a line as a string that is very well stretched and is very thin.
In Euclidean geometry there is exactly one line that passes through two different points.

Here we have two points, A and B, and the line that passes through them.

Note: Usually in formal geometry the point and the line and not really defined. They have only some proprieties (axioms) that are associated with them.