"Soldier-boi" is a porn video tag that refers to a young man or boy who is dressed as or portrayed as a soldier. This could include uniforms, gear, and other military-related attire or aesthetics. It may also imply a specific fetish related to men in uniform or roleplaying military scenarios.
In this context, "boi" is an informal term often used in the LGBTQ+ community to refer to young men or boys, particularly when combined with other descriptors like "soldier." This can add a sense of youthful vulnerability and playfulness to the scene, which some people find appealing.