If you're walking on a crowded street and you stop suddenly — abruptly and without warning — somebody walking behind you may bump into you.
If you experience love at first sight, you've suddenly fallen in love. If you're driving at night and a deer jumps out in front of you, you might suddenly have to swerve to avoid it. Suddenly is the kind of adverb you might use a lot if you're describing the plot of an action movie, a mystery novel, or any story in which lots of unexpected things happen.