This fluid is the environment in which the cell lives. The blood exchanges materials with this fluid so it becomes a good environment for the cell.The blood provides this fluid with the cell's needs and takes the waste from it.
In this fluid there are WBC that keep checking the presence of invaders around the cell. The PH is constantly adjusted, the salinity, and the temperature as well. This fluid is the one where nutrients are dissolved so the cell can absorb them.