A monthly plan adds credits to your account every month. Use them on any track whose label takes credits. You can still buy single tracks with money any time.
A track costs its price divided by the value of a credit, rounded up. With credits worth $0.50, a $1.49 track is 3 credits.
On a plan, the buy button turns into a coin with the number of credits. Press it and the track is in your Purchases at once, in every format the label supplied. Your balance shows in the top bar.