MixPad has a built in metronome which will tick along at the tempo and time signature specified in the project Tempo field in the mini-toolbar at the top of the project window. You can quickly switch the metronome on/off by clicking the metronome button in the same toolbar.
Pre-roll is a feature which allows you to start playback or recording from a position a set amount earlier than your current cursor position. This is most useful when you are recording and you want to hear the few bars before the position of your recording.
You can set a separate amount of pre-roll for both playback and recording in the Options dialog.
You can quickly enable/disable the pre-roll function by clicking on the button in the mini-toolbar at the top of the project window.