So what is it? How can I best utilise it? Any specific 5E wizards of the coast friendly API to get added?
API: Introduction As for specific scripts, it all depends on the sheet you are using and the way in which you like to play. There are scripts for anything from automatic hitpoint management, audio control, fx automation, teleportation, chat output modifiers, trap management and the like to sheet specific support scripts. I'd recommend taking a look at the github repository and see if any of the names catch your eye, they tend to be named after what they do so it is easy enough to find ones you think would be useful to you.
Once you have found scripts you think you would like to use, search the forums for their individual posts. Most scripts have a dedicated thread complete with 'how to' and syntax guides to get you started. It can seem a bit daunting at first but I recommend starting off with a couple of scripts and get comfortable using them before taking on more than you can handle... I currently run with 20+ scripts active on my main game at the moment :D
A good place to start is looking to see if your chosen character sheet has a companion script. The two most popular 5e sheets each have one, as do many other systems. The provide very useful automation that is tailor-made to the sheet (tracking ammunition, setting defaults, etc.) There are also several threads in the API forum wherein experienced users make recommendations of the scripts they find most useful. Some of them are very simple and straightforward one-trick-ponies, such as ChangeTokenImage, which allows you to automate swapping images on a rollable table token (so your wererats can all change shape at once, or so that your goblins don't all look alike.). Others are very powerful with lots of uses, such as Token-Mod, that don't do much off the rack, but allow you to all sorts of things that are difficult to impossible to do manually or for large numbers of tokens. For instance Token Mod could allow you to write macros that automatically grant darkvision to a token or turn torches on and off. They are API utility scripts, and it would probably be best to try some of the simpler ones first until you get an idea of what can be done. Eventually, you can look for synergies. For instance, I have a macro that invokes three different scripts in order to place and size a token image of a spell area of effect and play an associated sound effect, or another that shifts scene graphics and background music from a menu, in order to facilitate Theater of the Mind style play. This is the sort of thing that the API provides you: customization and automation. Don't hesitate to ask questions about any particular scripts, or if a Script solution exists for a particular problem you are having. The forum is great about helping folks.