Missed blocks = number of blocks that the server didn't sign off on.

This could be a result of many things, like for example when the blockchain froze last year I went from "0" to whatever the figure is today. On that day every witness missed blocks.

It could also be for example a simple error made when updating/restarting (aka HF).
Not taking the server offline immediately when doing this, a simple human mistake while doing the right thing.

If you do monitor the missed blocks "daily" and you notice that a witness server is missing blocks more frequently than others, then that could suggest that the server is not reliable or maybe a computer in a backyard garden shed or flooded mouldy basement. Just saying that to exaggerate a little, but then again, I have heard some horror stories about what all people use to "mine" crypto!

As for the "active" status.

on Steemian.info, if the witness/user name is in yellow, that means that the server has been offline since recently, which is not necessarily a bad thing, it could be just an update in question. In which case it would be best to send a DM to the person and ask what is up with that.

If the name is in red, then that server has been down for a while.
This is where a witness vote is going to waste.

If that witness becomes active again the name will change colour and I am sure they will notify their followers and friends that they have decided to get their witness campaign going again.

I hope this answer helps.

Cheers