I know what the effect of a filter on the sound of a synthesizer is, but I don't get wat the difference is between using a filter and filter automation.
Automation is a feature available in most digital audio workstations and many types of similar audio production software. Automation allows a parameter like volume, pan, or mute to be changed automatically during playback of a song by the software.
In the early years of multi-track mixing, engineers had to manually move faders, and turn knobs while recording the mixdown if they wanted to make changes during the mix. Sometimes the whole band, producer, and assistant engineer would help move faders during the mixdown because there were so many tracks and so many changes that were desired.
In the mid 70's, console makers started building consoles that could change the levels automatically using voltage controlled amplifiers (the same technology used for envelope generators in synthesizers). Now with digital mixing almost anything can be automated, including settings that are part of third-party plug-ins, like a synthesizer plugin.
So filter automation is the automation of one or more filter settings. The most popular setting to automate for a synth filter is certainly the cutoff frequency. That one setting is so popular for automation that the phrase "filter automation" really refers to "filter cutoff frequency automation".
When you set up the filter on a synth, you choose a single filter setting, including the cutoff frequency, and you leave it there (assuming you don't use an envelope). With automation, you can make the filter open and close during the song, which is a very popular sound.
Skip to 4:20 in this video to see how automation is done (in Logic Pro 9) and how it sounds.