It's a very common word, and you'll hear it used in various contexts, such as:
* "Tomar leche" (to drink milk)
* "Leche de vaca" (cow's milk)
* "Leche materna" (breast milk)
* "Leche en polvo" (powdered milk)
It's also used in some slang expressions, like "¡leche!" which is similar to saying "oh my god!" or "damn!" in English.