The simplest explanation for tire tread will be that they’re the ‘grooves’ in the tire that not just make it look good bit also perform the vital function of draining away water from under the contact patch. The larger tread of ‘knobby’ tires allows the knobs to dig into the soft ground and provide grip where the smoother tread pattern would not grip.
Talking of tread patterns, few know that tires have sea and land within. These are the technical names of tread features. The grooves are the ‘sea’ while the raised rubber surrounding them is the ‘land’. These land and sea patterns are usually unique to each tire model and most tire manufacturers favor a certain family of patterns compared to others.
Needless to say this land and sea pattern is quite usage specific for the tire. For instance off-road tires have very wide seas, narrow land and a high land to sea height to dig through and grip loose terrain like mud, sand and slush. Street tires have more land than sea and a low land to sea height. Dual purpose ones lie somewhere in between with wide blocks of land separated by moderately deep and wide sea areas.
There’s more to the trea if you consider a typical street tire. The center groove is there to remove water from underneath the contact patch when the motorcycle is running upright. All good with that except for the fact that the same groove when running over longitudinal irregularities (grooves or ridges) on the road tends to catch on to them and steers where they go. At times this can induce a severe wobble in the handle. Most tread patterns therefore follow a squiggly or zigzag pattern for the center line groove, thus retaining the benefits of good water clearance on wet roads and avoiding the grooving tendencies.