Moses was worried because the Israelites were asking for food. He tried to reason and think of possibilities to feed 600,000 men (+ their families). To his human perspective, it was definitedly impossible. However, God showed Moses that there's nothing impossible for the Lord.
It is interesting that Moses, the great leader and man of faith, goes to God and tries to convince Him that it is totally impossible to feed all those people. Moses even tries to look smart and give reasons why it is not possible (vs. 22). How many times have we done the same? We go to God and tell Him that it is not possible. Isn't He the God of the impossible? Didn't He say that everything is possible for Him? Isn't He the Creator? Then, why do we try to limit God with our limited mind?
I love God's answer to Moses' concern, which is an answer to my concerns as well. (vs. 23) "And the Lord said to Moses, Is the Lord's hand waxed short? Thou shalt see now whether my words shall come to pass unto thee or not." The way God provided food for the Israelites was so great and magnificent that it showed how powerful He is.
God wants us to never forget that everything is possible for Him. Even when it looks impossible to our human perspective. We do not need to worry, God has his own powerful ways.