To overuse something is to use it too much. If you use your cell phone way too often, texting your friends all day long and playing online Scrabble all night, you overuse it.
You'll drive your English teacher crazy if you overuse the word like, and you'll rack up all kinds of extra fees if you overuse your iPad. Overuse is also a noun (with a slightly different pronunciation, just as the noun use sounds different from the verb use) — extravagant or excessive use is overuse. The world's overuse of fossil fuels contributes to the rapid warming of the planet.