The photoperiod is how many hours a day your aquarium light is on; keeping it consistent and not too long (a timer helps) is one of the simplest ways to control algae.
Fish do not need bright light, and long or erratic lighting mostly benefits algae. A timer gives a steady day–night rhythm and takes the guesswork out.
In planted tanks the photoperiod is balanced against nutrients and CO₂; in an unplanted tank you can keep it modest.