In simple terms, the filters we use for visual astronomy reduce the amount of light that comes through the eyepiece. Filters never increase the amount of light. However, by reducing the level of light in one part of the light spectrum the filter may help to bring out another part of the spectrum which will show us the detail we might not have otherwise noticed.
This is the opposite of the way we normally think when it comes to telescopes. We are normally trying to capture as much light as possible so we can see the most detail or the dimmest objects. But, by removing light we can sometimes see more detail.
This is done in a variety of ways to suit the specific use case. Telescope filters are typically full aperture, covering the front of the telescope, or are eyepiece attached filters that screw onto the eyepiece.
Since filters reduce the light that reaches our eyes, the more aperture your telescope has, the more light it gathers, the more useful filters can be. If your aperture is small, a filter may remove so much light that you see no benefit.
What are large and small apertures in ...