it is a technique used to raise the expsoure value of a low zone. You place the "filter" in this case white plexi over the lens and expose a piece of film usually zone I to Zone II. Then following with a normal exposure without the "filter". It is an option used sometimes with subjects that have a high contrast range and one does not want to pull the development .
Check Ansel Adams writings for more detailed information.
Most commonly used with sheet film, but can be used with roll film, if the camera will allow making a double exposure.
In the old days, it was very common for press photograhpers to expose sheet film on clear days. they would just shoot the sky and then put the holder away for a time when the lighting conditions were very dim and use the pre-exposed film which of course added density to the negative, but no detail, helping the printing with thin negatives.
