Invest means they are investigating it for possible development. The numbers 90-99 (recycled after 99 back to 90) are assigned so as not to conflict with any actual system that may be out there while still keeping them in the two-digit realm (good for model runs and such). The letter after the number designates which basin the invest is in -- L is for Atlantic, E is for the East Pacific, W is for the West Pacific, and S is for the Southern Pacific. There's one for the Indian Ocean too, but I don't think it is I.