Wikipedia has a detailed explanation.

Here's my understanding in my own words.

1. Be impeccable with your words

Let your words be truthful to reality and let your actions be truthful to your words.

2. Don't take anything personally

I am completely independent of the good or the bad opinions of others. Anything anyone does is out of their own personal worldview.

3. Don't make assumptions

Assumptions about what others are thinking or what might happen in the future.

4. Always do your best

What is the highest truth and what is my highest wish? Embody them.