To overpower is to overcome with greater strength. In order to win an arm-wrestling match you must overpower your opponent.
The word overpower is often used in situations where great force is used or required against some person or thing. One army might overpower another, or a mugger might overpower his victim. But delicate things, like the scent of a perfume, can also overpower you, especially if they're too strong. Our emotions can also overpower us: we cheer after a home run and swoon when it's love at first sight.