A lot of free fonts online are labelled as “free for personal use” or “for personal use only” and can only be used on documents/designs/invitations/social media graphics which are made by you or by someone who is making it for you for free. If you're trading services or paying someone for their time, they need to use commercial use fonts. Some examples of personal use fonts are:

A wedding invitation you design
A friends baby shower invitation you design
A book report/essay/paper for school
Your personal blog/website (as long as it doesn’t include things like a portfolio with a link where people can pay for your services).