Rashi on Exodus 16:13/רש"י על שמות ט״ז:י״ג

1השליו THE QUAILS — a species of fowl which is very fatty (cf. Yoma 75b).

2היתה שכבת הטל THERE WAS A LAYER OF DEW — Consequently the dew was lying upon the Manna. In another passage, however, it says, (Numbers 11:9) “And when the dew came down [upon the camp at night, the manna fell upon it]” and so there was dew under the Manna! Thus we see that the dew fell upon the ground and the Manna fell upon it, and then dew fell again upon this, and so it was as though it were carefully packed in a chest (Yoma 75b).