Introduction This mishnah discusses how they would preserve the purity of the children of priests so that they could perform the red cow ritual without ever having become impure. This was an extra stringency due to the high degree of gravity with which they took the red cow ceremony.
Courtyards were built in Jerusalem over rock, and beneath them there was a hollow which served as a protection against a grave in the depths. They would not build the courtyards directly over the ground just in case there was a grave deep in the ground, and the dead body's impurity would rise and defile the priests above. The hollow between the rock and the building platform would serve to capture the impurity and prevent it from rising. This was a concept we learned much about in Tractate Ohalot a space the size of one handbreadth by one handbreadth prevents impurity from rising up above the space.
And they used to bring there pregnant women, and there they gave birth to their children and there they raised them. They would bring pregnant women there to give birth and raise their children there so that the children would never become impure. Again, this is not strictly necessary but it demonstrates the extra degree of severity with which they treated this purity ritual.
And they brought oxen, upon whose backs were placed doors, and the children sat upon them with stone cups in their hands. The following two sections describe how they would draw water from the Shiloah spring without possibly coming into contact with a source of impurity. The children would ride oxen down to the spring. The doors on the oxen's backs would prevent the children from overshadowing (making an ohel over) any source of impurity. They used stone cups because stone cannot become impure.
When they reached the Shiloah spring they got down and filled the cups with water and then they ascended and sat again on the doors. Rabbi Yose said: each child used to let down his cup and fill it from his place. They filled their cups from the Shiloah spring which is on the southern side of Jerusalem (also called the Silwan). Rabbi Yose says that they didn't even get off the backs of their oxen to do so. Again, this was an extra stringency to make sure they did not become impure.