Filters are tools which provide an alternative way of sculpting, because they do not rely on a brush radius. Instead they will affect any vertices that are visible and not masked.
The strength is controlled by click & dragging from left to right. The position of the cursor can be used to only affect specific areas, if auto-masking is used.
Many of the same brush types are also available as a filter type. This way much of the mesh can simultaneously be smoothed, colored or have some cloth simulation applied.