Usually, you decide what to look at and what to pay attention to. You are the actor. If you don't decide to look at something, you don't notice it. "It jumps out at you" expresses a sensation you feel, when your attention is "caught" by something. You were turning the pages of a magazine looking for an interesting article. You didn't want to look at ads. You weren't paying attention to the ads. You were just turning pages. Suddenly you turn a page and you see an ad that is visually surprising. You feel a sensation of surprise and shock, much as you would feel if someone hidden suddenly jumped out at you. You experience an "OMG" moment.

It means that somethings stands out in comparison to everything else. It is easily noticed, or it grabs your attention. For example, if four people were wearing white shirts and another person were wearing a red shirt, then the person in the red shirt would really "stand out" or "catch your attention." It's something that's unique. "It really jumps out at you," is basically a stronger way of saying the same thing.

"That's a very effective advertisement - it really jumps out at you." It really CAPTURES you/ it GETS the viewers/it makes you WANT it. Basically it means "That's a very effective advertisement - it really makes me want to buy it"