Pop-Up Ads can be found in The Nexus and Motherlessboard. By default they hide in the ground until a Vault Hunter comes close enough, and then they pop up out of the ground, most likely impeding the Vault Hunter's progress. They only spawn in fixed locations, so once Vault Hunters are familiar with their placement, they can be relatively easily avoided, but they are most often placed along paths likely to cause annoyance. Pop-Up Ads can either be "landscape" (wide but short), or "portrait" (tall but thin).
Pop-Up Ads will play a quiet, jaunty little tune while they're active, which is only audible when quite close. Once the Vault Hunters move far enough away, the Pop-Up Ad will retract back into the ground, and after a short delay it will be triggerable again. The specific ad is chosen at random each time the ad pops up, so the same ad location will usually show different ads each time it's activated.
Some Pop-Up Ads can actually be interacted with to purchase the items they advertise. For instance, the Pop-Ups advertising health, ammunition, and grenades can all be activated to dismiss the ad and receive some of the specified items, for a fee.
One Pop-Up ad features a "Hyperion Ad Gate," which promises to prevent Pop-Up Ads for non-Hyperion products from activating, but it is not actually purchasable. Pop-Up Ads also feature a red "X" in their upper-right-hand corner, shooting/meleeing the X will dismiss the Pop-Up.