I have been reading guides where people are suggesting a power level of X before doing some mission. What does this mean?
Power level is based on the gear you have equipped on your character. The better your armor and other gear, the higher your power level will be. If you want to increase your power level, you need to find better gear. To do this, you need to do more missions to obtain it. Power level sort of works as a way to prevent people from doing missions that are beyond their strength without having to put a character level boundary in place.
Use this guide if you want to know more about increasing your power level in Destiny 2.