Rashi on Exodus 16:8/רש"י על שמות ט״ז:ח׳

1בשר לאכל FLESH TO EAT— but it does not add as it does in the next phrase לִשְׂבֹּעַ, “to satisfaction”; the Torah thus teaches us a rule of conduct — that one should not eat meat to satiety. And what did He see (what reason had He) that He made bread fall for them in the morning and flesh at evening? Because the bread they asked for was a proper thing to demand since it is impossible for a person to exist without bread; but meat they asked for improperly, for they had abundant cattle, and besides it was possible for them to exist without flesh. On this account He gave it to them at evening, at a time of (when it would cause them) trouble, a manner which was not favourable to them (Mekhilta; Yoma 75).

2אשר אתם מלינים עליו means [YOUR MURMURINGS] WHICH YOU MAKE others who hear you murmuring MURMUR AGAINST HIM.