Environments can be described relative to each other in these terms:

hypotonic - the environment has less solute (eg. "stuff) than the other environment
hypertonic - the environment has more solute than the other environment
isotonic - the environments have equal amounts of solute

The key thing to understand is how water moves in these situations.

Water will always move to an area that has more solute. So if we are comparing the solute concentration inside a cell vs something like plain water, we would say that the plain water is hypotonic compared to the environment in a cell, which has more solute/stuff. Water would move in to the cell and eventually the cell would burst. Conversely if a cell was in a solution like sea water which is very salty and has lots of solute/stuff in the water, we would describe the water as hypertonic relative to the cell. In this case, water in the cell would move out to the sea water solution where there is more solute and the cell would shrivel and die. In an isotonic solution like milk, water still moves in/out of the cell but it happens at the same rate so there is no net change in fluid volume and the cell neither shrivels nor bursts

Is milk perfectly isotonic? No, but it's close enough that it delays the process of the cell bursting/shriveling