Dependency rules are a very powerful feature. This feature allows one Job to affect the scheduling rules of other Jobs. The best way to describe it, is to use an example. Say you have Jobs C and B that run every day at 8:00am, but they must not run until Job A runs before them. And Job A normally runs at 2:00am. If for some reason Job A fails to run at its expected time, Jobs C and B must not run till Job A completes. Dependency rules allows for this type of behavior.
A Job can be associated to up to three Dependency Rules. If a Job's schedule is impacted by a Dependency Rule (that prevents it from running on a scheduled date) a message is display in red that is shown next to the next run date to indicate the Job's schedule is suspended until the dependency rule is satisfied. If you put the cursor over the suspend message, a tool tip will indicate which dependency rule(s) are suspending this Job from running.