The combat is based on Roguelike movement, where you go and then everyone else goes. This removes the player reactions completely from the equation, and allows characters to move different speeds whilst still giving the player time to think.
It works in classic Roguelike style by giving each character 'turns' in quick succession, which each replenish 10 energy. Once energy reaches 100, the character gets a chance to act. Moving one square or attacking will cost between 80 and 120 energy, depending on how heavy your weapon is. Several characters may move before you get a chance to move again.
A character that's twice as fast as you will have two turns each time you move. But it's rare a character will be that fast. If a character is only 10% faster and they make the exact same attacks as you (different moves take different amounts of time) then they will get a free attack every 10 times you both attack. At the moment, you can tell how close a character is to a new turn by checking the circle underneath them (this UI may change).
It's being able to both react and move faster. Your weapon weight will make you slower, but your strength will offset this effect. Your reflexes will make you slightly faster (but not very much).