Can I travel through a country not covered by my pass?
No, your Eurail pass only gives you access to the train networks within the countries where your pass is valid. You're not covered in any other countries, even if you remain on the train for the whole time and don't change trains in that country.
If you want to travel in a country not covered by your pass, you need to buy an additional ticket for that section of the trip. Buy this ticket locally at a train station. Buying tickets on the train is often more expensive and you risk getting a fine.
You plan to travel from Munich (Germany) to Venice (Italy) with a Select Pass - 3 countries, valid in Germany, Italy and Switzerland. The fastest route is through Austria, which is not included in your pass. There are 3 possible solutions:
Order a different Eurail pass, one that is also valid in Austria.
Buy an additional ticket in advance at a German station, just for the part of your journey in Austria.
Research an alternative route, to stay within the countries where your pass is valid. You could avoid Austria by traveling through Switzerland, though this would take you longer time.