Link: https://en.wikipedia.org/wiki/Ballooning_(spider)

Ballooning, sometimes called kiting, is a process by which spiders, and some other small invertebrates, move through the air by releasing one or more gossamer threads to catch the wind, causing them to become airborne at the mercy of air currents and electric fields.

Spiderlings ballooning in the Santa Cruz Mountains of the San Francisco Peninsula

They can FLY!!?!?

yes. they are infiltrating your house as we speak :)

It's an unmistakable posture. I get so excited when I see a young or small spider "assume the position". They run around frantically and when they find their launch point they raise their abdomen and if you're lucky can see the silk they release and the next gust of breeze takes them away