Let’s keep this simple and say that there are two kinds of beliefs: positive vs. negative.
“I’m a healthy person,” is a positive belief.
“I’ll never amount to anything,” is a negative belief.
A belief is something that you accept as an unconditional fact. It doesn’t matter whether others agree. All that matters is that your brain thinks: THIS IS A FACT.
Many of your beliefs are private. You may go about your day acting confident and secure, but if deep inside you feel like an inadequate imposter, your belief is that, “I am an inadequate imposter.”