The concept of a marketplace of ideas refers to

the competition between varying opinions and ideas that are aired in public forums.

Explanation:

Modern societies are made up of different groups of people who have their own way in looking at the current societal condition's and problem.

In the past, people resolves such difference with violence. But, as we progress, we realize that we can solve the differing opinion by choosing a representative form each group and debating the ideas in public forums. and let the people who watch it form their own judgement Such forums is what referred to as marketplace of ideas.