Kick is mostly a newbie offensive skill, a step up from ‘push‘. Some higher end offensive skills may or may not benefit from being skilled in kick, info on skill synergy is a bit vague. In any case, use kick in combat only if you don’t have better.
Kick is also useful for damaging objects. You can for example use kick at a money box until it breaks open. This may damage any potions or similar items inside such boxes though, and might also trigger any traps on the box.
The ingame help reads:
Skill duration: 2. Type of skill : offensive skill. Party rows : 1st. Affecting stats: Str/Dex. It uses 15 endurance points. While most use their hands, or items wielded in their hands to damage their opponents, one can also make very good use of his feet. A well placed kick can do a variety of damage, such as knocking one's foe to the ground, breaking his ribs, or even crushing his windpipe. Those who are very skilled can defeat many skilled fighters with their hands tied behind their backs.