An elevated version of bro, so you say it when you want to be a tad cooler
Also doing 2 definitions in one so your welcome, you can also use the word broski on someone you like being friends with but also kinda like instead of using bro and other typical "friendzone" word.
Boy 1: Dude did she just call you broski
Boy2: yeah
Boy 1: that's a very good sign dude