This word has many meanings. We'll focus on some of the useful figurative ones.
When an injury sidelines an athlete, it puts that player out of the game and onto the sidelines of the field or court.
Figuratively, then, to sideline people or things is to make them stop participating, or to give them a less-important role.
And, if you're on the sidelines of some situation, you have an unimportant role in that situation, usually just watching instead of actually participating.
Part of speech:
Both a verb ("to sideline someone")
and a noun ("he's on the sidelines").
Other forms: sidelined, sidelining; sidelines
How to use it:
To use that first meaning, talk about injuries, problems, and issues that sideline people. Or, talk about people, events, and situations that sideline priorities, possibilities, and other people.
And to use the second meaning, talk about people who sit on the sidelines, who watch or observe from the sidelines, who are forced to the sidelines, etc.
examples: Extremists took over the state's history curriculum, rewriting the textbooks and sidelining the more progressive founding fathers.
Earning an advanced degree requires adding to the literature, contributing something, participating--it can't be done from the sidelines.