A sill plate is typically 2x4 or 2x6 treated boards sitting horizontally on the top of the foundation wall. It is anchored to the foundation block, and the floor joists are fastened to it. A common mistake by builders in older homes was to use a non-treated board. The sill plate is the first board to touch the foundation and is very susceptible to moisture wicking into the board from the foundation. This excessive moisture in the sill plate will cause it to rot.
Once the seal plate starts to rot, the floor joists will sink into the rotted plate causing the floor to pull away from the wall. In some cases, the whole structure will start to drop with the plate. Homes with brick ties will have the walls stay in place just from being held up by the ties.
The sill plate rot is most commonly noticed in your home by a gap in between the floor and the baseboard trim. Experienced professionals should be used in this repair, as it requires the floor structure of the home to be lifted up off of the foundation to replace the sill plate board. It is not a simple tasks, and there are many repairs that are not sufficient to pass a home inspection. The entire plate must be replaced. It cannot be wedged or beamed.