hold weights just fixes the weight on your bone so that it can't change
so if you hold it at 0.8 and adjust the other weights on the other bones that influence that vertex then it stays at 0.8 while the others change

flood changes the weight of the entire array of verticies on the mesh to the value of the weight you've selected.
so if your weighting value that you will paint on is 0.8 then you click flood then all possible verticies will have a weight of 0.8 for that bone