Solidarity is one of those terms that gets thrown around a lot, a term you’re just expected to intuitively know. And while its definition is in many ways intuitive (”unity or agreement of action”), it has a further meaning, which comes from its origins in early leftism.
We all face different struggles, and we all face our struggles differently. Being in solidarity with another’s struggle doesn’t mean that you have yourself lived it, but that you struggle with them, that you fight together against a shared foe. This idea has had widespread use through the centuries, and has been widely used by both anarchists and Marxists.