Short answer
They should discuss matters that can materially affect married life, including residence, finances, work, children, family responsibilities and religious expectations.
Islamic Guidance
What Islam teaches
Honesty and fulfillment of agreements are important in Islam. Clear understanding before marriage can reduce disputes later.
Practical Guidance
What you can consider
Discuss expectations openly and respectfully. Important disagreements are easier to identify before marriage than after it.
Sources
Qur'an 5:1; Qur'an 4:19.