Cognitive Behavioural Therapy is one of many different approaches to supporting not only people with poor mental health but anyone seeking to improve the way in which they interact with people and the world.
The charity MIND gives a nice explanation of CBT here: https://www.mind.org.uk/information-support/drugs-and-treatments/talking-therapy-and-counselling/cognitive-behavioural-therapy-cbt/
Whilst CBT is used in psychotherapy, there are lots of self-help resources that use the CBT approach too e.g. here https://cogbtherapy.com/introduction-to-cbt.
And a student has recently found the CBT Workbook for Dummies helpful in dealing with their depression and anxiety https://www.dummies.com/health/cognitive-behavioural-therapy-workbook-for-dummies-2nd-edition/.
I have a copy in Office 20 if you would like to have a look anytime.
