Those are not goggles, this is the visor! In SP, you tag an enemy while pressing F (listen to the nanosuit, boy!), in MP, you only have to cross over your future victims. The advantage of tagging is, that, in SP, you can see enemies through walls, and in MP, guess what? You can see enemies through walls! And besides, you see them even in stealth mode and if a tagged enemy gets killed, the tagger gets extra points.
And yes, the number in the top right corner are the kills/or whatever for each team. The red one if the one for the enemies, the other one is your team.