It's the opposite of "revolutionary"; when the ruling class in a country opposes revolutionary movements stay in power.

When a capitalist state (ie. ruled by the capitalist class) is in decline (rising costs, worsening working conditions, etc), workers tend to unionize and organize into political parties with the goal of overthrowing capitalist rule.

Capitalists react to this (reactionary) by shifting the blame onto already marginalized people to scapegoat them so they can stay in power, which leads to fascism if capitalists manage to stave off a working class revolution; hence why socialists say fascism is capitalism in decay. This is what happened to the Weimar republic for example and is what led to Nazi Germany.

• Socialism for Beginners

• What Exactly Is Liberalism (no, it's not about being "woke")