What does signing cofounders agreement mean?

I saw people are signing cofounder agreement without including lawyer. I guess they use it to be „safe“. I have some doubts about it and I need some explanation. This agreement seems non valid, it seems like people are signing them to be sure they can work on a project.

If I sign cofounder agreement what does it mean? Does the person who sings it get the equity no matter what? Is that agreement valid legally? What if I or other person wants to terminate it?

I'm talking about signing it using online service like DocuSign or similar