Tevul Yom, Chapter Four, Mishnah Two
A woman who is a tevulat yom may knead dough, cut off the hallah, and set it apart, and she should arrange it on an Egyptian basket, or on a tray, and then bring it near and call it by its name. For it [the dough] has third degree uncleanness and third degree uncleanness is clean in hullin.
The woman is a tevulat yom (the feminine form of tevul yom). When she touches the dough, it will have third degree uncleanness, which is not unclean when it comes to hullin. The problem is that if she touches the hallah, the part of the dough that is removed and given to the priest, she will disqualify it. Hallah is holy and therefore a tevul yom touching it would defile it and a priest couldn’t eat it.
To prevent the hallah from being defiled she first separates it but doesn’t yet call it hallah. Then she puts it on a type of tray that can receive impurity or an Egyptian basket which also cannot become impure. Then she has to bring the other dough near and then, without touching it, she can call the hallah by its nameHallah! This way she can give pure terumah to the thankful priest. Pretty cool, huh?