When we are born and begin entering society, we adopt several agreements with others about how things are, how we are, and how the two relate. Over time, we adopt more agreements based on feedback from others. Often, these agreements serve to control us and limit our independent happiness. I am not good enough, I need to be successful to be valuable, I need to be (appear) smart.
The Four Agreements offers this model, develops it, and then gives four agreements that are available to adopt that replace most other agreements we have that limit us. They are: Be impeccable with your word, Do not take anything personally, Do not make assumptions, and Always do your best. Each works together to liberate negative judgements of ourselves and others that we carry around.
This is the sort of book that, for me, works, but is annoying. The writing isn't great — it feels judgemental the whole way. Its model is surely incomplete, but... the high-agency it offers is wonderful. It's short and easy to read. I put it alongside other books like Existential Kink. Useful model, but the book itself is quite difficult at times.