Mohamad
It means to leave/give some money to someone for a service that they have provided you which is more than the cost of the service. For example, a taxi journey might cost £10, but you pay £12 if you were very happy. The £2 is a tip for the driver. We often leave a tip for the waiter in a restaurant. We pay the bill then put a little money on the table for the waiter, (usually around 10 percent of the cost of the bill in the UK).
Victoria