Filling in gaps between pavers with a caulk-like material?
It could very well be an outdoor sanded caulk, meant to seal the gaps in the pavers to prevent weeds and water, which can disturb a nice level surface of pavers over time. I mean, if you'd trust a 1/2" bead of general-purpose silicone caulk to seal flashing on your roof, I would trust it to perform similarly in most any other application.

It could also be a grout-like substance, used in much the same way as for tile. This kind of material would be more rigid, and I wouldn't count on it lasting very long if you get cold winters; it'll crack, loosen and eventually crumble away as the pavers, and the ground itself, shrinks and expands.