I'm currently reading Wittgenstein's tractatus and

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I'm currently reading Wittgenstein's tractatus and I don't understand what certain signs in combination with other signs mean.

Can someone explain 5.5321 to me.

"Thus, for example, instead of ‘(x): f x⊃ x = a’ we write ‘(∃x). f x .⊃. fa : ∼(∃x, y).f x . f y’."

Specifically what does the colon signify? I've seen it before in the tractatus but only in passages where understanding its meaning didn't seem that important to me so I thought it would suffice to look it up after I'm done with the book.