Once you reach level 10 and acquire the prayer skill (which is automatically gifted to you by accepting the final starter reward), you will be able to pray to the gods once every hour. By doing so, you will gain several nice benefits that making praying every hour worthwhile.
To pray, simply click the button at the top middle of your screen once an hour. You will know that prayer is available because the button will be lit up and glowing. Praying only takes a few seconds, and you will get a timer that shows how long you have left until you can pray again.
You will be rewarded with experience and a buff to a random statistic such as attack power or defense. Additionally, there is a small chance to be awarded rare items such as Lionheart scrolls that give you 30 minutes of double experience, or even powerful fortification crystals that let you enhance your gear.
You will only gain experience and item rewards for the first four prayers per day. Past the first four, you can still pray - but you will only get the temporary statistic buff.