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Explanation
The Joke
The comic depicts Spider-Man swinging through space, having shot a web that has attached to a flying saucer containing two bewildered aliens. Spider-Man (or one of the aliens) exclaims "What the... was that there when we left New York?" The joke is that Spider-Man, whose power is to swing between buildings using his webs, has somehow swung so far or so high that he has latched onto a UFO in outer space. From the aliens' perspective, they have suddenly acquired an unexpected hitchhiker.
The absurdity is that Spider-Man's web-slinging, which normally requires nearby skyscrapers to work, has apparently found the one "anchor point" available in the void of space: an alien spacecraft. The aliens are just as confused as he is about this situation.
The Humor
The humor plays on the fundamental limitation of Spider-Man's power set: he needs things to swing from. In Manhattan, there are always buildings. In outer space, there are not -- except, apparently, for a conveniently placed flying saucer. The comic imagines the logical extreme of what happens when Spider-Man's web-shooting reflex encounters an environment with only one available attachment point. The aliens' baffled reaction adds to the comedy, as they are presumably unfamiliar with the concept of a human in spandex latching onto their ship via a biological adhesive string. It is an April Fools' Day comic (published April 2, 2018), which fits its playful, crossover-style premise.