One of the meanings that you will find a lot on the internet and LGBTQ community is for a person’s sexual or gender identities according to Wikipedia.
However, the other meaning that we are interested in here is an alternate spelling for “boy”. This can have various reasons but the reason we are focusing is to distinguish that from real young boys.
What does Boi mean?
A boi is a male bottom (i.e. submissive partner), defined not by junior age, but by his obedient role. This is true especially when that implies him being subject to spanking and/or other punishment by the dominant “top”.
We can see an increase in using this term in Femdom scenarios when a mistress is dominating a beta male. And also when a top male is dominating a sissy boi (a crossdresser or a transsexual) in behavior or sexually.
Boi was, and is, an identity term, primarily used by people of color, that traverses the messy, occasionally overlapping space that encompasses masculine women, nonbinary transmasculine people, and transgender men. 
Furthermore, Among gay men, he might be a youngish feminine man, akin to a “twink.”