I'm not sure if there will be some introduction to what's happening when you start the campaign in Halo 5 but it's probably best to know some backstory. Go read a quick summary of the campaigns from previous Halo games if you can, and definitely watch the Spartan Ops movie on YouTube. The Spartan Ops movie is just all of the cutscenes from Halo 4 Spartan Ops combined into one big 'movie', but it has some good backstory that will definitely be present in Halo 5.