The Arecibo radio telescope detects radio waves with all kinds of bandwidths.
Narrowband signals are what we believe an alien civilization would use to communicate, whereas broadband events are most likely due to
natural astronomical processes. (It takes a tremendous amount of energy to send a broadband signal.)
Hence, SETI@home is interested in focusing on the narrow bandwidth signals (though it investigates broadband waves as well).
The SETI@home screensaver performs an averaging procedure called "baseline smoothing" that eliminates broadband noise and brings all the other narrow bandwidth events down (or up, depending on the size of bandwidth) to a common "baseline" level.
Also, sometimes the data from Arecibo as a whole gets slowly louder and/or softer over time. Baseline smoothing evens out this effect as well.