24h volume is what?

I'm looking to factor in the liquidity of a pair in order to determine if it's tradable, based on my particular criteria. bid-ask spread is one mechanism, but another is volume and that's what I think I'd like to focus on.

On Binance, in the Markets overview, it shows a 24h volume. I understand that's the rolling 24h volume indicator, not the 1d volume indicator. But, what is that a volume indicator of? Total unit quantity of that particular base pair that was traded? Total traded value of that particular pair? Total traded value of the base currency? Total trade value of the quote currency? Other?

Would the 24h volume be considered a reasonable barometer for liquidity of a pair?

Once I have an understanding of what that is, my intention is to query and/or calculate that from the API but I need to know what I 'm looking for first :)