A toll-free telephone number is a telephone number in which the person who owns that telephone number is charged for calls that come to them, rather than the person making the call to that number being charged. These numbers usually begin with "1-800" in North America (800 being the area code under the North American Numbering Plan), but are different around the rest of the world.
They are not to be confused with "collect calls", which are made when the person making the call does not want to have to pay their telephone company to make it. In this case, the person receiving the call has to agree to be charged for that call, while those owning toll-free numbers already agree to being charged for incoming calls.