a childhood friend

A "childhood friend" is someone who you were friends with when you both were young, like 10 years old or younger. It's usually someone who lived nearby, whose parents were friends with your parents, or whose house you stayed over at a lot.

a friend of mine

It sometimes sounds strange to say "my friend" when you're telling people about someone who's your friend. That's because it can make it sound like you only have one friend. So we also use the phrase "a friend of mine":

I'm meeting a friend of mine for dinner at around eight.

Here's a general guideline for whether to use "my friend" or "a friend of mine":

• If the person you're speaking to already knows which friend you're talking about, use "my friend".
• If the person you're speaking to hasn't met your friend, or probably doesn't know which friend you mean, use "a friend of mine".

And to help you remember, here's an example that uses both — first "a friend of mine" and then "my friend":

I went out to a bar with a friend of mine from college. We were talking at the bar and my friend asks me, "Hey, what's that black spot on your forehead?"