5021LC - 3V lithium button cell 55mAh

mAh/Ah (milliampere-hour/Ampere Hour)
This is the Capacity usually expressed in mAh /Ah meaning milli-Ampere hours/Ampere hours. This is basically the amount of current the battery will give multiplied by the number of hours that it will give it for, so a bigger number means that the battery will last longer in the device you are using it for.

Think of the battery like a bucket of water, mAh or ah rating is how much water the bucket can hold the bigger the bucket (mAh/Ah ratting) the more water it can hold.(current)