"In the context of systematic reviews and meta-analyses, a protocol is a document that presents an explicit scientific 'road map' of a planned, uninitiated systematic review. The protocol details the rational and planned methodological and analytical approach of the review." (Shamseer et al., 2015) In other words, it's an explanation of what you are going to do, how, and why.
Creating a protocol prior to embarking on your evidence synthesis review is critical to ensuring transparency and mitigating bias. At minimum the a priori protocol should be registered online, ideally in one of the preferred registries listed below. Researchers may also consider publishing their protocol as a peer-reviewed journal article.
Once your protocol has been finalized and registered, your team can begin the review. Deviations from the protocol should be avoided, but where necessary, should be documented in the protocol registry and explained in the future article manuscript.
There are established best practices for what should be covered in your evidence synthesis protocol. At minimum you should cover the elements outlined in the Preferred Reporting Items for Systematic review and Meta-Analysis Protocols extension (PRISMA-P). Additional elements may be recommended or even required by individual publishers, such as Cochrane and the JBI. In some cases, those publishers or protocol registries may make available templates that can be used to guide the creation of your protocol.
Evidence synthesis reviews must have a priori protocols, i.e. a protocol that was developed and published before work begins. What is in it for you, the researcher? Disseminating your protocol effectively plants your flag in the topic, indicating that you're working on the project. Researchers should not embark on a new evidence synthesis project until they've verified it hasn't already been done or is in progress. (Note: a topic may be researched again with good reason, such as methodological problems or outdated searches.)
The most common method of dissemination is to add your protocol to a protocol registry. A few recommended registers can be found below. For more options, check out this article: Where to prospectively register a systematic review (Pieper & Rombey, 2022). Some journals will publish protocols. If you want to pursue this avenue, verify if the journal charges article processing fees (APCs) for protocols.