Green open access (also called 'self-archiving') refers to making a version of work (usually the author's accepted manuscript) available in an open access repository.
These can be institutional repositories such as the Cambridge's Apollo repository or subject based, such as arXiv, PubMed Central, RePEc or SSRN. Green open access can be considered to be 'secondary open access'.
Depending on the journal or publisher, there might be different conditions attached to this 'green' route, such as which version can be used (often the accepted rather than the published version) and an embargo (a time during which the manuscript cannot be made publicly available after publication).
Placing work in ResearchGate or Academia.edu is not considered to be green open access and is not usually allowed by publishers. You should always check your publishers policy and your route to funder compliance. You can check publisher policies by using Sherpa/Romeo, and you can also contact the Open Access Team.