These shortcuts indicate the required equity needed in order to call a bet or not.
So for example if facing a 1/2 pot bet on River we need to have at least 25% equity in order to call.
You can calculate this manually like that:
Amount to call / (Amount to call + Total pot size)
Or if you can't do this in your head efficiently and quickly (like me)
You can simply memorize this:
https://imgur.com/a/G743ueQ
However if you get check raised these shortcuts no longer works and you have to do the whole calculation.