If you are new, spend some time with the Adobe User Guide.
Offset means that the value is offset. If you are using trim paths, offset will change the starting and ending position of the trim. If you set Start to 25 and End to 75, you can send half of a closed path around the loop. Offset values for trim paths are in degrees.
Offset does different things in different effects, but it always moves one or more parameters around their current values.