Making the source code available for everybody to see and use for derivative works under certain conditions like retaining authorship notices or keeping the same license (copyleft). There are many varieties of this, some are more permissive than others. But this does not directly imply a certain business model.

Some people do donation-based development. And that's fine. And some people do things entirely for free, that's also fine. (But don't be angry that somebody then takes your code and does what the license you put it under permits them to do, e.g. not pay you.)

And others are corporations that have full-time employees write code and publish it as open source. See all the Red Hat work. Or Intels and others contributions to the Linux kernel.

Open source is more than the lone dev in their basement doing selfless acts and sometimes begging for donations.