Experience or commonly known as XP or EXP, is a measure of how far a player has progressed in a skill level. After gaining enough XP a player will advance to the next skill level. EXP can be obtained by doing tasks related to the skill you would like to advance in.

As you level up you will be able to do more tasks which will award more experience but each following level will need more xp. 

Currently mobs give more xp the further away from Lumbridge they spawn. Animals now give a lot less xp than mobs even if they are far away from spawn. Only applies to melee fighting. Some skills show their progress while doing the task on the minecraft xp bar. As well as its level. Currently only applies to mining and a few other.