Okay, a 900W PSU will be able to provide up to 900W of power (as long as the manufacturer hasn't lied about its wattage).
The Bronze, Silver, etc ratings are the PSU's efficiency: how much power it drains and how much it provides. Basically, it measures how much energy is lost when it is being "processed" by your PSU.
A 900W PSU with a 90% efficiency would mean that it can output 900W, but to do so it'll be using up 1000W from your outlet. It's using more power than it provides, and the wasted energy is let off in the form of heat.
This of course means it's better to have a higher efficiency: it'll save in your power bill, and it'll reduce your heat emission.
the ratings go 80+<bronze<silver<gold<platinum. what it means is the percent of output divided by input. It can be taken 2 ways, good PSUs will give the true output, but will actually draw more than what they are rated for. Your PSU is perfectly fine, i wouldn't worry about it. 900W can run 780ti in sli.