Jobs offer you a way to make money. To start a job, either right-click the "Job Manager" NPC at spawn or use the command, "/jobs browse".
You'll get a menu with a list of different Jobs. Left-click the Job to see more information, such as the materials you can collect and turn in for money. Right-click the Job to take it.
To complete a job, simply collect the resources it requires. For example, if you've selected the "Woodcutting" Job, you'd chop down trees and turn in the logs.
As you complete Jobs, you'll gain experience and level up through the Job progression. This offers a few bonuses including increased Jobs rewards and the ability to purchase certain special items in the Jobs Shop with "/jobs shop".