What does it mean to have a two party system

what does it mean:a political system consisting chiefly of two major parties, more or lessequal in strength.

A two party system is a system in which only two parties are present and relevant in the politics: in this system, the power is usually alternating between the two parties.

In contrast, in multi-party system, there are more parties, who form coalitions upon winning the elections, if they don't receive enough seats (in a two-party system coalitions are practically never found).