What does $Y_i \perp Y_j | X$ mean in linear regression?

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So I saw in a text book that for linear regression model $Y | X \sim \mathcal N(X\beta, \sigma^2 I)$ we have $$Y_i \perp Y_j | X\ \ \textrm{for}\ \ i \neq j$$ I would like to ask what does this expression really mean?