Many open access journals have Author Processing Charges, whereby authors have to pay for their research to be published. Most also have waiver policies where they allow publication for free or at a discount if, for example, the authors are based in a low- or middle-income country.
This blog in Scholarly Kitchen provides a useful overview:
It includes a link to guidance on 'best practices for APOC waivers'
and a useful index of publisher APC waiver policies
Best wishes, Neil