Risk of Bias includes a variety of different subject related to Systematic Reviews, including Bias in the Search Process, and Bias in the Studies selected themselves.
In the search process, Risk of Bias includes Limiting Searches to a specific Language (excluding research due to language preference) or Time Frame. Additionally, a major consideration in searching the literature is finding research that is NOT published. Publishers have inherent Bias in that one intention of publishers is to publish content that is new, novel, or ground-breaking. Not all research accomplishes this. In fact, research that proves something is not true or emphasizes something that is already known, is unlikely to be published at all.
In the article selection process, Risk of Bias appears, as noted in the section on GRADE, in poor blinding of participants, poor allocation of study participants, randomization to study arms, incomplete data, selective reporting of data, and more.
For more information, see Chapters 7 & 8 in the Cochrane Handbook.