Generally, controlling the speed of something.
In wireless mobile networks, it usually means artificially limiting the speed of a data connection.
This may be done by the carrier (network operator) at cheaper pricing levels. (In that case, users must pay more for faster speeds.)
Carriers may also impose throttling after a user consumes a certain amount of data in one billing period. That first amount may come at full speed, but all data transferred after the limit is reached may be throttled to a slower speed until the end of the billing period.
Finally, some carriers will impose throttling on specific users when the carrier feels that those users are consuming "excessive" or unusually large amounts of data.