means getting used to new conditions. Drastic change in the environment will stress most living things. So fish need to be introduced into a different body of water slowly. As an example you can let the bag they are transported in float in the new pond until the temperatures equalize. You could also say that a pond needs to acclimate to a change of a larger fish population. The bacteria of the pond would need to ramp up to take care of the new waste. Rapid changes in pH, temperature, fish loads, etc. can cause stress to the organisms in a system.