The Irminsul was apparently a wooden pillar that played some role in the Germanic paganism of the Saxons. The name means something like "great pillar," irmin descending from ^* ermanaz in Proto-Germanic (the same word also appearing in Old Norse compounds like jörmun-gandr, "great serpent") and sul from ^* sūlō meaning " pillar," with cognates in many Germanic languages.

As for its significance: no one's really sure. It apparently was some kind of object of worship, if we believe the (typically hostile and always Christian) sources that mention it. Many early Germanic religions seem to have held religious functions in sacred groves or woods, and it seems that certain trees might have figured in these functions. Adam of Bremen described a sacred tree in Uppsala from which sacrifices were hung, echoing a much earlier description of Germanic sacrifice by the Roman writer Tacitus, and many "bog body" mummies were first hung before being thrown into the water. IA Christian king or missionary felling the pagans' sacred tree is a stock occurance in conversion narratives. It's pretty clear trees figured in cultic practices.

So what was the religious significance of this? It's impossible to say for sure, as our sources are so fragmentary. In Norse mythology (by far the best-documented of the Germanic pagan religions,) trees figure in many ways: the universe is structured around Yggdrasil, an ash tree which binds the various worlds together. In the aftermath of Ragnarök, the end of the world, the two surviving humans will emerge from a hiding spot inside a tree and repopulate the earth - there's some kind of rebirth imagery at work here. Odin was also hung from a tree for nine days before returning to life, neatly mirroring real-life human sacrifices as described by Adam of Bremen and others.

Unfortunately our sources on Saxon religion, and particularly that of the Continental Saxons, are far less complete than even the sources we have on the related Norse belief. Probably the best we can say with certainty is that some variety of worship centered around the Irminsul and that it may have involved some variety of sacrifice (though as this isn't my specialty, I can't state this is the case with certainty) and that Charlemagne had the Irminsul (or perhaps an Irminsul) cut down in the course of one of his many wars against the Saxons in the late 8th century. Beyond that, it's just informed speculation.

The Irminsul has nothing to do with the Goths, who spoke quite a different Germanic language than the Saxons and who originated far to the east.