Tellers are MPs or peers who count the votes during a division (vote) and then announce the result.
They are usually also whips.
There must be four tellers at each division - two to count the ayes and two to count the noes.
Each pair of tellers consists of one member who declares that they will vote aye and one who will vote no - to check each other's results.
If at the time that the division is called there are not four members prepared to be tellers then the voting will not take place.
For instance, if there are two tellers for the ayes present but only one for the noes then the Speaker will declare that "the ayes have it".