In any chess tournament, we have to have a way to limit how long each round will take. The time control is this limit. If your time control is G/30, that means that each chess player has 30 minutes to complete his or her portion of the game. The game could then last a full 60 minutes (30 minutes for each player). Our local scholastic in person events usually have a time control with a 5 second delay. That means, if a delay-capable clock is brought to the game by one of the players and used, each time a player takes a turn, the clock waits 5 seconds before subtracting any time from that player's allotment. In a 60 move game, using 5 second delay could then mean that the game might take an additional 10 minutes to complete.
Online games are usually a bit quicker. A time control of G/15 +5 would mean a base of 15 minutes for each player's portion of the game. However, each time a player completes a move, 5 seconds is added to his time. So each 12 moves gets the player an additional minute of time. It's much harder to run out of time with a 5 second increment.