hii okay so
a .psd is normally what you call any file in a Photoshop format, but on tumblr when we refer to psds we usually talk about a bunch of effects combined together that can be applied to a picture or gif to enhance and make it look clearer/more or less colourful, depending on your editing style. a psd takes quite a while to make because you should build it in order to suit almost any image or video you’re gonna use it on, and sometimes it can have up to like 10-20 layers or so. for example, mine consist of a lot of brightness/contrast effects and a very lowly saturated colur scheme.
long story short, to create a psd you basically take a picture and start editing the fuck out of it until it looks the way you want it to, then merge the layers of every effect you’ve added on it into ONE layer and that layer is the psd. you then save it .psd format so that you can later open it in Photoshop and apply it to any other gif or image
as for the additional brightness or contrast layers, those are only necessary if the image or video is really shitty and the psd is just not enough to make it look pretty. you also can’t include them in the psd from the start because then it would be too much for all the other “normal/not so shitty” videos and images.
hope i made this clear for you!