Filtration is the act of separating a heterogeneous mixture of a solid and a liquid. If the solid particles are small enough they will remain suspended and evenly distributed in the liquid when mixed. This is called a suspension. If you run the suspension through a filter such as a filter paper or a sintered glass funnel, the solid particles will remain on the filter while the liquid passes through the filter, thus separating the two. But this can depend on the nature of the filter. Filters are usually rated by a value called the porosity. This is the smallest size of particle that the filter is capable of capturing. Anything smaller will simply pass through with the liquid. Since suspensions usually consist of very small particles, it's important to have a properly rated filter for capturing particles of this size. It should be noted, however, that if multiple different solids are present in the suspension, the filter will not be able to differentiate between the two and the captured solid will remain a mixture of solids.