SR621SW is the name of a small 1.55 volt battery.  It is a popular battery for watches and other small electronics. 

S stands for its silver-oxide positive electrode terminal.
R stands for its round shape.
6 stands for 6.8 mm diameter.
21 stands for 2.1 mm height.
S stands for its sodium hydroxide electrolyte.
W means the battery meets the International Electrotechnical Commission (IEC) standard for watch batteries.
Capacity: 20 mAh
Voltage: 1.55V

Where to Buy
Brand names such as Energizer and Duracell will have their own names.  They use 164 or 364 for their identical battery names. They can be found in many drug stores and department stores.

How to Save Money
It is possible to substitute the alkaline version of the battery which costs less.  AG1 is the common name of most alkaline versions of the same size battery. The capacity, however, is less. AG1 batteries have a capacity of 13 mAh.