Trim path

To add on to what Jose said.

A path is either open or closed. An open path is a line, it has a recognizable start and end point. These points are based on the order in which the vertices (anchor points) of the path are created, first, second, third, etc. A closed path is a shape, think a circle or rectangle. The first vertex of a closed path looks different from the others, it usually has a hollow square around the smaller vertex square.

In After Effects the first vertex of a path can be changed by selected a vertex and choosing Layer > Mask and Shape Path > Set First Vertex (or right-clicking to get the same menu). This would allow you to control where on a path the Trim path efffect starts so that you can control the look of the animation.

Another use of the trim path Start and End properties is if you want to animate the visual movement of a line (usually just a stroke) along a path. 

By changing the Start and/or End properties and animating the Offset property the stroke will travel along the path from beginning to end. 

Think that every path has an start vertex and end vertex (the first and end points you draw). Start and end parameters in Trim Path refer exactly to this. If start and end are 0 and 100, a complete stroke will be shown. If start is 20, the first 20% of your stroke will be trimmed. If end is 80, then the last 20% of your path will be trimmed. If End is lower than Start then what is happening is that the stroke will be shown in reverse direction. Hope this help