PlayPerfect gives you the ability to control the tempo, or speed of the piece you are practicing. For example, it can be very useful to slow the piece down when you're first learning it, and gradually speed it up as you get better.
The current tempo affects the rate of playback, and the speed of the metronome. It is shown towards the top-right of the window. It is given in Beats Per Minute (BPM), which most of the time will be quarter notes per minute. So if the tempo is 60 BPM then you should play one quarter note per second; if it is 120 BPM then play two quarter notes per second, and so on.
To change the tempo click on the Change Tempo button, or on the tempo display itself. This will open the Tempo dialog, in which you can choose a new tempo. To return to the piece's original tempo, click on the Ideal Tempo button. When you can play the piece at this speed without making any mistakes, you're ready to move on to the next piece!