Devil's Pool is basically an area you can swim to that's right on the edge of the falls. It's a great place for Instagram photos
I didn't get a chance to visit Devil's Pool as it was the wet season when I went and the water level was too high
From my research, Devil's Pool is usually open between late-August to early-January during the "dry" season, even during these times it's not guaranteed as it depends on the water level. Only when water level is low enough would it be considered safe enough
Also, it's not possible to visit the pool on your own. It's part of the Livingstone Island tours that are offered by various operators there (like this one on Tripadvisor). The cost of these tours range from lower US$100 to over $200
An alternative to Devil's Pool is Angels' Armchair, which is a similar experience. Again, it's only offered as part of an organized tour and only available mid-July to late August, and early January through mid February