An electric trimmer/shaver like this is a cordless grooming tool designed for trimming or shaving facial hair, beard, body hair, or sometimes even nose and ear hair. It uses an electric motor and blade system instead of a traditional razor, making grooming quicker and usually less irritating to the skin. 
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What Does “400 mA” Mean?
The “400 mA” (more accurately 400 mAh) refers to the battery’s capacity — how much charge it holds. This is a key part of understanding how long the device runs between charges:
• Battery Capacity (400 mAh):
This value (milliampere-hours) tells you how much electrical charge the battery can store. A 400 mAh battery can, in theory, provide 400 milliamps of current for one hour before it runs out.