“I can't carry a tune at all.”
You're at a karaoke bar with some friends. One friend asks you to sign up to sing a song with her, but you tell her you are a terrible singer.
I can't carry a tune at all.
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The phrase "not at all" means really, completely none. It's a very extreme way of saying "not".
The word "not" can also be replaced by any negative word - "no", "don't", "nothing", "isn't", etc.
There was no one there at all.
I didn't say anything at all for the entire trip.
Being able to "carry a tune" means that you are able to sing pretty well. Here's how to use "carry a tune":
A: Are you a good singer?
B: Yeah, I can carry a tune.
In this example, the second person is being a little modest. That's because "I can carry a tune" doesn't mean that you're a good singer. It just means that you can sing decently well.
If someone is a really bad singer, say that they "can't carry a tune":
Allen, a good singer? No, he couldn't carry a tune to save his life?