The Puppy Dimension is a pocket dimension that so far has one known gateway to enter/exit it from. That gateway is a magic item known as the “Pocket Portal To Puppy Dimension1). It was acquired by Walker Florida Cleric. Once per day, it can be used to enter the Puppy Dimension2)
The Puppy Dimension is vaguely thought of to be like what you'd encounter if you went through a portal and emerged into the middle of the annual Puppy Bowl. You enter and immediately get overwhelmed by cute puppies all needing pats and cuddles while having too much energy and being overly adorable. Since time spent there is so limited, one never gets tired of being in the Puppy Dimension.
In practical use, the Puppy Dimension/Pocket Portal to it is a way to escape a situation for a few minutes. Suppose you need to vanish from combat or a social scenario just long enough to escape danger. The solution is simple. Go pat some puppies.
If there is another way to access the Puppy Dimension, or more elaborate details about it, no one is aware of it yet.