A "no-brainer" is a decision that's very easy to make. You are able to decide immediately without thinking about it.

For example, if someone offers you a job at twice your current salary, accepting the job might be "a no-brainer". You can talk about it like this:

It was a no-brainer. I said yes immediately.

You can remember "no-brainer" by imagining a decision that's so easy to make, even a person without a brain can make the right choice.