The y-intercept is the point on the graph of a function (equation) where it crosses the vertical y-axis. This applies to all functions, not just linear functions. It's found by evaluating the function when x=0.
In linear functions there are always two intercepts: a y-intercept and an x-intercept (they may be the same point, the origin). If the intercepts are not the same (the function doesn't pass through the origin (0,0)) there is a non-zero constant in the definition of the function. A non-zero y-intercept is the constant term.
A linear function (equation) is represented graphically as the extension of a line passing through both intercepts.