Lets go over the preview panel. This is located in the upper right  corner of your screen.

This preview panel gives you options such as moving through your movie. A project is made up of frames just like in real video footage. In After Effects, moving within a project is referred to as moving down the timeline. This timeline is represented in units such as frames, seconds, and minutes. This preview panel allows you to move forward or backwards frame by frame. Pressing the play button let's you preview your project. You can also use the ram review, which gives you the ability to watch the project at a faster speed. You can also hit the space bar to preview your animation.