Normal definition of projects will be like some work plan, which is well organized.

At freelance sites, projects mean online jobs.

This sentence “Buyers post projects” means that buyers outsource jobs.

Projects are simply the online jobs that one can get at freelance sites.

Buyers are the people who post projects where service providers are the people who place bids on these projects.

There are many types of projects at freelance sites. For example there are data entry projects then there are copywriting projects etc.

The work that we can get at freelance sites: instead of calling it online jobs or simply jobs, we call it projects.