Definition: to maintain the existing opinion, position, or status; to resist
Examples:
The employees are resisting the changes. We need to train our team managers how to hold the line regarding this restructuring plan.
Our competition is attacking us hard on pricing, but we need to hold the line. If we drop our prices too, there will be a big price war and we will all lose.
By the way, this idiom was inspired by my Australian friend Serpil Şahin. Thanks, Serpil!