The Mishnah continues its list of the stringencies that apply to sacred foods but not to Terumah.
"Ochlin ochlin neguvin b'yadayim mesoavos b'Terumah, aval lo b'kodesh" - one may eat dry foods with unclean hands in the case of Terumah, but not in the case of sacred food.
Dry foods may be eaten with impure hands when they are Terumah. This does not mean that the food happens to be dry at the moment, but that it has never come into contact with liquids that would render it susceptible to tumah. Since it is not susceptible to tumah, even if impure hands touch it, it does not become impure.
With sacred food, however, the law is different, because with sacred food we take into account the concept of chibas hakodesh, the endearment of the sacred: chibas hakodesh renders the food susceptible to tumah even without any contact with liquid. Therefore even dry sacred foods may not be eaten with impure hands, since the hands render them impure.
The Gemara's question and the Rambam's explanation:
The Gemara asks that this principle - that chibas hakodesh renders sacred food susceptible to tumah, and therefore sacred food may not be eaten with impure hands - is too obvious for the Mishnah to need to teach it. Various explanations were therefore offered as to the Mishnah's intent, and we will explain according to the approach of the Rambam.
According to the Rambam, the Mishnah is dealing with a case in which the person does not touch the sacred food at all: someone else places it in his mouth, or he places it there himself with a fork. The Mishnah teaches that even in such a manner the sacred food may not be eaten, out of concern that at the same time he may also put into his mouth ordinary food that has been rendered susceptible to tumah, touch it with his impure hands and render it impure, and it will in turn render the sacred food impure.
With Terumah, by contrast, we are not concerned about this, since Terumah does not require the extra care that sacred food does. We therefore do not suspect that he will touch moist ordinary food with his hands and that it will then affect the Terumah, and Terumah may be eaten with impure hands - as long as the Terumah itself is not susceptible to tumah, or as long as someone else places it in his mouth.
"Ha'onen u'mechusar kippurim tzerichin tevilah l'kodesh aval lo l'Terumah" - an onen and one who still lacks atonement require immersion for sacred food but not for Terumah.
An onen - one whose close relative has died and has not yet been buried.
One who still lacks atonement - a person who was impure, and even after immersing in a mikvah and waiting for nightfall, still has to bring his offerings.
Both of them require an additional immersion in a mikvah in order to eat sacred food, even if they were already pure beforehand, but they do not require this immersion in order to eat Terumah.
The reason is the concern that they may have become impure: an onen, in his distress over the loss of his relative, may well have become impure; and one who still lacks atonement, since until that point he was forbidden to eat sacred food, may not have been careful enough and may have become impure. They are therefore required to immerse again before eating sacred food, but not before eating Terumah.
In summary: in this Mishnah we learned two further stringencies that apply to sacred food: eating dry foods with impure hands, which is permitted with Terumah and forbidden with sacred food because chibas hakodesh renders it susceptible to tumah - and according to the Rambam's explanation, even when one does not touch the food at all, out of concern for a mixture of ordinary food that has been rendered susceptible; and the requirement that an onen and one who still lacks atonement immerse again for sacred food but not for Terumah, out of concern that they may have become impure.