Ball-tampering is one of the many ways a fielding team sometimes tries to do to gain an unfair advantage by changing the condition of the ball in illegal ways.
The fielding team tries to alter the condition of the ball which will help it move and trouble the batters. There can be many ways to tamper with the condition of the ball such as roughing it with sandpaper, use mints, boot spikes, or if you are brave enough then bite it with your teeth.
The punishment which ICC has decided for this offense comes under law 41, in which five runs awarded to the batting team and if the team desires the ball can also be changed by the umpires. Depending upon the level of offense, the accused player(s) can be banned for the following few games as per the interpretation of the match referee.