3.0 is the number of audio channels it supports.
In this case, it reproduces the left, center, and right front channels
if connected to a source delivering a surround-sound signal.
The surround channels, if there is any signal for them,
are "folded into" (mixed with) the left and right front channels.
The zero means it doesn't reproduce low-frequency effects (LFE)
which are designated as ".1", meaning that there is one channel
and it covers one octave of the audible spectrum.
In highly sophisticated setups, you might find a designation of ".2" for stereo LFE.
You would be well advised to learn the basics of home theater audio.