There can be two types of soft spots in wood floor system.
Floor Sheathing:
The sheathing (plywood or OSB (oriented strand board) or the structural floor support system, the wood joists. Is the soft areas are in the plywood or OSB sheathing it's probably because it's been getting wet. You will typically find this problem in your wet areas (kitchens, baths and water heater areas). You can just cut out that plywood or OSB (make sure the saw blade depth-of-cut is only slightly more than the thickness of the floor sheathing, usually 3/4 of an inch or a little more) in that area and replace it. If it is under a framed wall you may have to remove and replace that wall too (be careful to make sure it's not a structural support wall before removing). After you cut out the soft of deteriorated floor sheathing you will be able to see if the water damage has also deteriorated the structural floor joists.
Structural Flooring System (floor joist)
The only other type of soft flooring is a problem with the structural floor joist system. There are only really three real possible causes of this. The joist got wet and have deteriorated (see discussion above regarding the floor sheathing), something VERY heavy fell and broke a joist (unlikely) or the floor joist system is not sitting on the piers correctly. If any of these have things have happened the joist system can be fixed in that area by replacing the affected individual joists (or splicing or sistering another joist onto the damaged one). Or resolving any problems at the joist/pier junctions. These floor joist repairs can get to be an involved carpentry job.