Metronome
To hear on the metronome tap the icon lower left on the Play screen and turn on the Tick button.  The volume control underneath lets you adjust the tick volume relative to the music.  The sound the metronome makes depends on the current time signature, with strong beats emphasized over weak.
Count-in
If you are using PlayScore 2 as an accompanist, you may not want the app to start playing until you are ready.  This is the purpose of the Count-in feature.  Count-in will tap out one or more measures before the music starts, giving you time to pick up your instrument.
Tap the Count-in icon 2nd from left at the bottom.  Tap+/- to set the number of measures to be counted.  If the music has a pick-up bar or anacrusis, Count-in will also tap the beats prior to the first notated beat.
While one or more measures of Count-in are set they will be played before the music starts wherever you tap in a score.
(Apple devices only)