Over simple answer here.
Low water flies can be two things: First just a smaller fly (hook size) or it can be a 'larger' hook with very little in the way of material(s) used in the tie.
High water flies will usually be fairly large/larger hooks/etc., as you want the fish to really 'see' the thing as it moves into his zone of vision. Just personal observation here (only) but the more 'coloured' the water the higher the probability the fly will be a 'dark' tie to better contrast with the water. Sounds backward, but it works ..
"We sleep soundly in our beds because rough men stand ready in the night to visit violence on those who would do us harm." – Winston Churchill