An optical filter is transparent material used to select the wavelengths of light allowed to reach a photosensitive material. Stained glass would be an example of a filter. A red piece of glass would absorb all other wavelengths of sunlight except for the red it lets through.
For WFC3 and many of Hubble's instruments, a single filter from a whole set is positioned to receive light before the detector (photosensitive material).
The filters are switched out mechanically to allow only the desired wavelength of light to hit the detector.
The images released to the public are usually combinations of single-filter images (done in post-processing of the data). For instance, single blue, red, and green filter images could be combined to create a full-color image (TV's and monitors also combine these 3 colors to achieve full-color). Click here for an excellent tutorial from the STScI on how colors are produced in Hubble's Images.
Filters are made of transparent and thin metallic materials that are carefully designed to meet the specifications of astronomers and scientists. In addition to filters, there are also grisms, which are optical devices that work somewhat like a prism. It has the effect of spreading the spectrum of light across an image plane. So it also controls what wavelengths hit a detector.
Each filter or grism goes through extensive processing and testing until the desired specifications are met. Among the most important ones are:
Two filters are represented in the graph below. Filters are designed with variation so that scientists can study specific wavelengths of light a stellar object might be producing.
Actual performance measurements of filters are not quite as clean-cut as what is shown above. Below is a more accurate representation (black wavy line).
The full version of the image above shows all the filters and grisms of WFC3's two channels. Their central wavelength and bandwidth are represented pictorially. The filters/grisms are named according to their central wavelength and bandwidth in some cases.
They are also named according to the element they might be calibrated for. For instance the filter HeII would be sensitive to wavelengths of light that Helium produces.
The following links are designed to provide scientific information about WFC3 to the general astronomical community. They provide access to the instrument's exposure time calculators, documentation, and other topics of interest to astronomers wishing to observe with WFC3. Many of the links are provided courtesy of the Space Telescope Science Institute (STScI).