Throttling is a process that ISPs use to artificially limit the speed of a connection.
There are several reasons why an ISP might throttle an individual connection, but the most common reason is when a customer goes beyond their data allowance.
If this happens, your ISP may throttle your connection until your data resets at the end of the month.
Specific online activities that ISPs frown on, such as torrenting (file-sharing), might cause your ISP to throttle your broadband connection.
Throttling is also used more generally for management but that often results in faster internet for customers than if the connections weren’t throttled – similar to how traffic lights improve congestion by slowing down cars.