Read it as "when facing a bet of x% size they must be bluffing or value betting a worse hand y% of the time for it to be a break even call"

Or you could have a draw that has enough equity to win that % of the time. Unless its the river of course.

These types of charts confuse the piss out of me honestly. I do the same mental flip flopping your do and confuse myself. I always think about it in terms of min defense frequency or bluff catching indifference. If you take a bet divide it by the total pot including the bet then that then take 1- that number this is how often you need to call so your opponent is indifferent to bluffing you with any two cards. I.E. $50 bet $100 pot 1-($50/$150) =.66. If you want to know how often you need to value bet with a given bet size in order to keep your opponent indifferent from bluff catching your bet you just take 1- their pot odds. I.E 1-($150:$50) or 1-(3:1) or 1-(.25)=.75