Experience rates, or exp rates refer to the hourly rate at which experience can be obtained through training a specific skill. They are directly related to the amount of exp that is gained via performing certain actions relevant to the pertaining skill, with some skills having favorable exp rates and others having very unfavorable ones. The speed at which experience can be gained in any skill is a large determinant in classifying how "easy" or "hard" the skill is to train, and thus directly determines how many players chose to spend time in training the skill. Although it is worth noting that a skill being labelled as easy or hard, is subjective. One might simply say that some skills are less tedious to train than others.
Some of the quicker and easier skills to train (such as Cooking, Fletching and Thieving) can get experience rates at an upwards of 200k per hour, whilst some of the slowest and most tedious skills to train (such as Slayer, Runecrafting and Agility) get maximum experience rates between 20-60k per hour, making them seldom trained in comparison with the quicker and easier skills.