Anytime means at any moment or whenever it's needed, without restrictions on when something can happen. If your teacher says you can ask questions anytime, she means during class, after class, or whenever you're confused. When a friend says “come over anytime,” they're telling you their door is always open.

Compare it to a specific time: if someone says “come at 3:00,” you need to arrive exactly then. But “come anytime this afternoon” gives you freedom to choose what works best for you.

People often use anytime as a friendly response when someone thanks them, like saying “you're welcome.” If you help a classmate understand a math problem and they say thanks, you might reply “anytime!” to show you're happy to help again whenever they need it. Some people write it as “any time” (two words) when they mean the literal sense of “at any time,” and “anytime” (one word) when they mean “you're welcome,” though many writers use the one-word version for both meanings.