Tinning is what you might call "pre-soldering", applying a thin layer of solder to the individual pieces to be joined so that when you put them together and apply the iron again, the solder flows together easily giving a neat complete seam. This means, particularly if the pieces are big, you are only trying to heat half the total metal involved and its quicker and easier.
Tacking, as Jim said, is just a few blobs of solder to hold things temporarily. So yes, you're right, it is holding things temporarily, but using solder, not other things
In my piece about signal ladder construction, I said I tinned the rail length on one side, folded it in half and then tacked at 2 or three places to hold it. Subsequently using the iron again to split the tacks. The tinned sides then made it much easier to solder the rungs to the sides.