If two players differ in strength, they can (and should) agree that their game will start with additional "handicap" stones for the weaker player. Handicap stones are always given to black, and -if chosen well- compensates for the difference in Go skill. With handicap stones the game will become more interesting and each player will have an equal chance to win.
 
On a 19x19 goban the handicap is equal to the difference in ranks. For other goban sizes Dragon scales it appropriately.
 
Traditionally, handicap stones are placed on hoshi in a set pattern depending on the number of stones. On Dragon Black is allowed to place handicap stones freely.
 
Please note that many players prefer to have handicap stones placed on the hoshi points. So if you're the one getting the handicap it's good form to agree wether to use traditional or free placement with your opponent. Offering a game in the waiting room let you choose if you prefer standard handicap placement.