Money laundering is the process of hiding money acquired through criminal activities.
Criminal enterprises often deal in cash, but trying to bring large amounts of suspicious cash into a bank can be very noticeable to law enforcement. So, many criminal enterprises attempt to "launder" or "clean" the money through a variety of ways.
One way that money is often laundered is through a business front. In the business front, a criminal may use cash from the illegal activity to help run the business. For example, a drug dealer may set up a check cashing business and cash the checks with illegally obtained cash. This would be an example of money laundering. Money laundering is often used in large-scale organized crime. It is illegal and those convicted of it can face jail time or other consequences.