If $u(\xi=0+, \eta)=u(\xi=0-,\eta)$
Does this mean
$\lim \limits_{\xi \to 0+}u(\xi,\eta)=\lim \limits_{\xi \to 0-}u(\xi,\eta)$ ?

If $u(\xi=0+, \eta)=u(\xi=0-,\eta)$
Does this mean
$\lim \limits_{\xi \to 0+}u(\xi,\eta)=\lim \limits_{\xi \to 0-}u(\xi,\eta)$ ?

Yes, that would be the meaning, if I should take a guess - especially if it makes sense in whatever context you've seen it in.