Running a journal is expensive. There are production staff, editorial staff, IT demands (organisation and publishing software), and other office costs. There are also reviewing costs - while the main subject matter is often done by volunteer reviewers, ceraint parts of the review may require paid specialists (eg. A medical journal may need to hire a mathematician to check the statistical analysis).

Traditionally, the way journals were funded was by selling subscriptions to individual scientists or universities and libraries. This is still the case, but there are now so many journals that it is inpossible, even for top universities to keep up. I teach at a med school and while some of the most famous med journals are subscribed to and the library has a login, minor or specialist journals often are not available, and while the library can get a copy it usually costs $20-30 for the request to be sent out to a partner library who does have a subscription and get the article back.

Increasingly, many journals now offer an option where the authors pay to have their article published. So, if yoi write a scientific article, send it to a journal and their reviewers and editor accept it, then you can pay the cost of publication (usually around $1000) and the publisher will make the article "open access (free to anyone).

As $1000 is very cheap compared to a new scientific experiment or study, this is easily affordable by anyone who had the money to do the study. Indeed, many charities and governments which give out money for scientific work, now specifically include a publication fee in the donation, and make it a requirement that any articles published come out "open access".