If you’ve seen Robin Williams in Dead Poets Society (or read the last lesson), you’ll know that carpe diem means seize the day. It’s a common sentiment in literature and in YOLO tattoos.
Carpere literally means to pluck. Carpe is the imperative form, giving an order or instruction. Diem is the accusative of dies, or day. The whole phrase is an instruction to pluck the day while it is ripe, rather than wait around for an uncertain future. This is even clearer in the original source, a poem by Horace:
carpe diem, quam minima credula postero
Pluck the day, trusting (credula) as little as possible (quam minima) in the future (postero).
Credula is feminine here because Horace is addressing a woman.