With Lithium cells it is important that the voltage of a cell is not going beyond a limit, in a serial connection with not that equal cells there is always one that will reach the limit 1st. If that is not taken care of then the voltage will go higher than wanted. A balancer will switch on a small load on that cell and drain some capacity while still charging the other cells.

Most simple chargers have a 300mA balancer current but with higher current chargers you have a higher current or even adjustable balance current.

There is no specific current but balancing will go quicker with higher balancing currents when charging with higher charging currents.