TL;DR: two methods (cosmic microwave background, and cepheids) are used to measure the rate of expansion of the universe that give different results. The "Hubble tension" is this disgreement about this value. Recently a new method using the tip of the red-giant branch (TRGB) gave a value that is in the middle of the other two.
How big is the difference? How many percent? And how big are the error bars for each method?