What it means is that they've judged that you are disabled enough, and have few enough resources and family assistance, so that you need SSI payments to survive.
The one-year interval is what they call a "reevaluation". Almost all cases of disability payments have reevaluation periods. You get disability payments for that time, and then you go in for another evaluation--by a doctor, usually, or a psychologist--to confirm that you are still disabled.
There are variable periods for reevaluation; generally, one, three, or seven years. Seven years is for things that are obviously going to be permanent, like a spinal cord injury that took place five years ago with no recovery of function since. The one-year period means that they believe that in the future you may no longer be disabled enough to need SSI, so they want to check up on you every year. Most of the time, it's a formality and you're approved for another time interval, the same or longer.
Chances are, they figure that you're disabled, and you need SSI; but you have the potential to improve to become more independent and eventually to support yourself. For most autistic people, I think that's probably the case. Learning useful skills can make you employable even when you can't hold what NTs would consider "simple" jobs.
There are ways to stop the reevaluation. The main way is to get into a pre-employment training program or a supported employment program. Those postpone your reevaluation. The idea is that they want to keep you fed and housed while you learn to support yourself, with the goal of eventually getting off SSI altogether. This is what I'm doing; I'm working with my state's bureau of vocational rehabilitation to get a college degree, learn independent living skills, and become self-supporting. If it weren't for that, I would be on a three- or five-year reevaluation cycle, I think--not sure which, right now.
In general, the BVR can get you employed within anywhere from six months to six years. Six years is the extreme because it represents going through a four-year college degree slowly--and yes, they do send people to college, if they determine it's the best way for you to get sustainable employment. Ironically, college may be one of the less expensive ways to get someone employed because of how most disabled people who are eligible for BVR help to get through college are also eligible for lots of scholarships, both for being disabled and for being academically talented enough to get good grades consistently.
So--Yes, you were approved. The one-year interval just means that you, like the vast majority of SSI recipients, aren't approved for life, but are periodically reevaluated to be sure that SSI is still necessary.