In early spring, when the water temp is in the mid 50's, fish will start moving up. However, they won't just go from deep water to beds in one day, they take their time.
In the winter, bass will hang out around deep structure. As Spring approaches and they begin to think about spawning, they will start to relate to cover more so than structure. (Structure is the underground geography, cover is brush piles, weeds, docks, ect.)
As they migrate from deep to shallow, they will travel along river channels, ledges, and bluffs, (structure) stopping to rest and feed along any cover they'll come across. These various forms of cover at various depths are what I consider their stages.
For a real life example, last weekend I was catching fish off of brush piles in 25 feet of water. This weekend I found them hanging out in stumps at 15 to 20 feet. Within a couple more weeks, I'm guessing I'll be able to catch some males up shallow roaming around looking for bedding areas.
While that first wave of bass are moving up, there's a second wave behind them. And a third wave behind the,, ect. One of the reasons I love spring fishing is because there are so many different things you can do to catch them. Around my area, once late March comes around there's fish guarding fry, fish on beds, big females nearby beds in deeper water, fish are still moving up and staging, they are just everywhere and it's wonderful.
Hopefully that clears some things up.