What is a party system and how does it relate to the concept of an “electoral realignment?”

A party system is a concept in comparative political science concerning the system of government by political parties in a democratic country.

Party realignment explained
A central component of realignment is the change in behavior of voting groups. Realignment means the switching of voter preference from one party to another, in contrast to dealignment (where a voter group abandons a party to become independent or nonvoting).