What does Accumulate mean?

Accumulation means that when multiple coins are inserted, the value of the coins is added together. 

Example:
The coin machine accepts 1 euro coins and the set time per coin is 15 minutes. 

When a user puts 1 euro into the coin machine, they get 15 minutes of time. If they put 2 coins into the coin machine, they get (2 x 15) 30 minutes of time, if they put 3 coins into the coin machine, they get (3 x 15) 45 minutes of time, etc.