In combat, combatants must move around and use abilities asynchronously. All actions take some amount of time to complete. Combatants must also reactions to the events of the battlefield, or be left helplessly performing meaningless actions.
For Example: At the start of combat, pulse 0, Sir Autobahn Diamondtrout III begins to cast Bee. The spell has a 5 pulse cast time, so he can neither move nor attack nor take any other meaningful actions until the spell has completed. On pulse 5 the spell triggers, and he is able to immediately begin another action. If he chose to cast Bee again, the second spell would trigger at pulse 10.
Attacking also takes an amount of time in combat. This time is determined by a combatants base windup time, and is modified by the weapon that they are wielding, plus or minus any additional modifiers. Attacks must be declared at the start, and then take the entire focus of the character for the full windup time.