it is a chrome extension that has a back end (in the options page) where you can select from a very large swath of D&D spells, creatures to polymorph into, feats and magic item effects,
in order to create a limited list of things your character can use that fit in a frame injected into a window of an existing site, where you can then finetune your choices (what's active now, what power attack level I have chosen, which creature I'm polymorphed in and using which spell) and can return a list of all effects (new HP value,, AC values, Saves...) in a specific format, a string that the site can …