Timers are used to run script at regular intervals. You can set up timers by selecting "Timers" from the tree menu.
The "Interval" box lets you specify how much time elapses before this timer "fires", in seconds. If you select "Initially timer is on" for any timer, that timer will be "ticking" from the moment the game starts. If you don't select this option, you can turn the timer on using the "Turn on a timer" script command in the "Timers" category.