A filter is any oscillator used to delete some but not all of the spaceships in a stream. An example is the blocker, which can be positioned so as to delete every second glider in a stream of period 8n+4, and can also do the same for LWSS streams. Other examples are the MW emulator and T-nosed p4 (either of which can be used to delete every second LWSS in a stream of period 4n+2), the fountain (which does the same for MWSS streams) and a number of others, such as the p6 pipsquirter, the pentadecathlon and the two blockers hassling R-pentomino oscillator shown under factory.
Filters can be used to make guns of very high periods without crossing two guns, for example, the period-20 glider gun can be made p40 by adding a blocker, then p80 by adding Rich's p16, and even p160 by adding 68P32.1: