I thought about booking a tour for part of a trip, but they all say "from x dollars." What does this mean? It is never explained.

If you click through they show a specific cost. If so, why do they say "from." Is this the actual price? Does "from" mean that your price can be more if they don't fill the tour?

In pricing ads, "from" means "at least this and probably more, sucker."

No, they don't raise the per person cost if under-subscribed. Its because . . . tours cost different amounts for different dates. You read the tour description, read the itinerary, look at the hotels if they are listed and see a "From" price. Then you pick a date and there is the price for that date

And then there is often a single supplement. What it means is "this is the lowest possible price for one person traveling as half of a couple".