It's perfectly clear to me
People like (for a bit of fun) to estimate their dogs' equivalent ages / life expectancies by the 1 person year = 7 dog years tradition.
This is broadly inaccurate because larger dogs typically have shorter lifespans than little dogs, so this image provides a less inaccurate way to estimate the dog's age etc. Take the dog's age in calendar years - the green numbers, and read out towards the outside for the dog's comparable age in human years to the right number according to the size of the dog.
So, a large dog aged 7 years equates to 54 years in a human's lifespan.
It's all not to be taken too seriously and is a bit of fun. I found it interesting, Bob