I’m relatively new to Dynare myself, so hopefully someone will correct me if I make a mistake.
“varobs” is used to define the variables for which you have data, and want the estimation to take that data into account. The variables listed under “varobs” do not have to be endogenous variables from your model, they only need to have data available to be included in the estimation. One example would be oil import prices in a small open economy - you can use the WTI oil price as a varobs in your model, but it would be declared under varexo if you didn’t have data for it.
I hope that quick explanation helps!
Ned has another simple explanation here as well.