Following the Exodus, God brought His nation to Mount Sinai, and commanded His loyal emissary Moses to prepare them to receive the Torah:
Thus shall you say to the house of Jacob and declare to the children of Israel: You have seen what I did to the Egyptians, how I bore you on eagles’ wings and brought you to Me. Now then, if you will obey Me faithfully and keep My covenant, you shall be My treasured possession among all the peoples. Indeed, all the earth is Mine, but you shall be to Me a kingdom of priests and a holy nation (Exodus 19:3-6).
All the people answered as one, saying, “All that the Lord has spoken, we will do (na’aseh)!’” Later, they expanded this commitment and proclaimed, “All that the Lord has spoken, we will do (na’aseh) and we will listen (nishma)!” (ibid. 19:8 and 24:7). In other words, the Israelites’ great faith in God led them first to agree to perform all the mitzvot even if they did not understand their significance. Later, they agreed to study the Torah in order to understand the meaning of the mitzvot. Ever since, the expression “na’aseh ve-nishma” serves as a declaration of faith and a statement of identification with the divine mandate.