Nationalism is the belief that people who share a common language, culture etc. should be free from foreign rule. It can also be used to describe the the extreme attachment or patriotism to ones nation.
Nationalism became a problem for Empires like Russia and Austria both prior to and after WWI, as the countries they controlled began to demand independence. This weakened the state due to their reliance on these countries resources and funds. It also created internal problems, such as civil unrest, for their respective governments during an already tumultuous period.