It's a pretty nebulous term the roots of which are in Lenin's Imperialism in which he describes a class of workers that sides with imperialists because they receive a portion of the superprofits of imperial exploration.

Historically it was used to describe business union leadership or union workers more broadly, especially those in the war industries. Further left tendencies tend to broaden the scope of who is a labor aristocrat including professional workers or even the whole of the white working class.

Personally I think as Marxists we need to reckon with the fact that many workers in the imperial core have petite-bourgeois brain worms precisely because their minds have been thoroughly rotted by debt and consumerism subsidized by the blood and sweat of the global periphery. That doesn't mean we should eschew work among these workers but we should understand it's limitations.

For instance there's a massive naval shipyard near me that employs many people (mostly cis white dudes). They have a militant union and excellent pay and benefits. Unsurprisingly they're all ridiculously conservative and nationalist and I've had multiple employees there tell me war would be good for them because it would mean more work and better compensation. Just because many of these workers are union proles does not mean that they will easily align with the interest of the global proletariat and if we are organizing or agitating them we need to understand that.