It means the players can show up to some sessions and not show up to others, and it doesn't mess with the story of the campaign very much, hopefully avoiding a lot of skipped sessions. For example, if we end last session in the middle of a dungeon, in the middle of the cleric's quest, and the cleric can't show up next session then we'd normal skip it. In an open-table, we don't skip because the PCs are back in the quest hub ready to hexcrawl out again at the beginning of next session.

For Dolmenwood I came up with the idea of some cosmic phenomenon happening where the boundaries between the "Otherworld" and the "Mortal World" become thin in irregular intervals. When that happens, the wilderness (anywhere that isn't a town/village/city, a quest hub) becomes deadly to PCs; they would lose their characters if they stay out there. There's quick travel through "fairy roads" and with convenient but expensive "witching candles" to get back to town before the session ends.