"And He called to Moshe; and Hashem spoke to Moshe from the Ohel Moed" (Vayikra 1, 1). The Medrash deduces from this pasuk that Moshe did not enter the Mishkan until Hashem specifically invited him to enter. This attribute was a manifestation of da'as. The Medrash goes on to say that a talmid chochom who lacks this attribute of da'as is inferior to an animal carcass.
What exactly is da'as and why is it so important? Rav Wolbe (Pirkei Kinyan Da'as pg. 77) explains da'as as an awareness of what one is doing as opposed to actions done out of habit or mechanically. With regard to davening, da'as is referred to as "kavana." We all know that it is quite possible to recite the entire Shemoneh Esrei while our minds are totally occupied with thoughts that have nothing to do with the tefillah. Such is a tefillah recited without da'as.
The same thing holds true for everything that we do and say. People converse with others without thinking about what should be said and what should not be said. As a matter of a fact, an entire day filled with events, conversations, and mitzvos, can pass without one paying attention to all that he has said or done!
"Yishuv hada'as" is the description for actions performed with an awareness of what is being done. Someone possessing yishuv hada'as lives an entirely different life than one who is lacking this quality. His davening and performance of mitzvos create a connection with Hashem, his conversations are meaningful and his actions show the thought behind them.
Da'as is the quality possessed by Moshe Rabbeinu and one that everyone must strive to attain. This is a quality so vital that its absence causes even a talmid chochom to be considered inferior to a carcass. A little focused thought can go a long way.