Albert Barnes Commentary Psalms 106:7

"Our fathers understood not thy wonders in Egypt; They remembered not the multitude of thy lovingkindnesses, But were rebellious at the sea, even at the Red Sea." — Psalms 106:7 (ASV)

Our fathers did not understand - They did not fully comprehend the design of the divine dealings. They did not perceive the greatness of the favor shown to them, or the obligation to obey and serve God under which they were placed by these remarkable manifestations.

Your wonders in Egypt - The miracles performed there on behalf of the Hebrew people.

They did not remember the multitude of your mercies - The great number of the divine interpositions on their behalf. They did not allow them to influence their conduct as they should have done. The aggravation of their offence in the case here referred to was particularly in the multitude of the mercies. It would have been sinful to have forgotten even one act of the divine favor; it was a great aggravation of their guilt that so many acts were forgotten, or that they failed to make an impression on them.

So now. It is a great sin to be unmindful of a single favor conferred by God; it is a great aggravation of guilt that men live continually amidst so many proofs of the divine goodness; that they are fed, and clothed, and protected; that they breathe the pure air, and look upon the light of the sun; that they enjoy the comforts of domestic life, the blessings of liberty, and the offers of salvation; that they lie down and rise up; that their toils are crowned with success, and that the blessings of every land are made to come around them—and yet they forget or disregard all these proofs of the divine mercy.

But provoked him at the sea, even at the Red Sea - (Exodus 14:10–12). They rebelled against him. Even amidst the wonders occurring there, and after all the blessings which they had received at his hands, when they were in danger they doubted his power, and called his faithfulness into question.