Art boards are a page or layout container within Illustrator. A variation of layers, you can add multiple items such as paths and type to it. You can have multiple entries. They have their own panel. You can create new ones, add, move, export etc
You can find the art board tool in the tools panel and also the panel via the window menu. There are also a set of commands available via the 0bject menu where you can convert a path, rearrange, fit to artwork bounds and fit to selected art.
By default, when you create a document you always have one. There has to be one but you can have many (there is some upper limit)
When you create a new document you can go to the number of boards field to push this up to 10 or 20 etc and they will appear when you click create.
When you have selected the tool, the control bar will add some new functionality such as options for the current board (height etc) plus a little + and you can click that to add a new one. It will be a straight copy of any current selected one and it will have a basic name 1, 2, 3, 4 etc but you can rename that. You can also resize it using the tool as well as move etc. Go to the far left and there are a lot of presets to choose from and the board will be set to one of those such as apple watch, iPhone etc
You can re-size the selected item. You can do it interactively with the tool or go to the top control bar and enter the X and Y and H and W etc for it and you can also go to the options and change the settings there also. You can't rotate them or have them as circles etc.
Select the tool and select the board and move it, you can have it at the side, above, on top and overlapping etc with an existing one etc. You can also go to the object menu and the related commands and use the rearrange command and the panel will give you options for the layout as well as order and also the columns and also the spacing between them.
You can select them and then hold down the alt / option key and drag and this will duplicate the object as well as the items contained such as stars and type etc. You can also do the same with documents. If you have a document with a board, select it, drag it to a new document and it will add that to the new document with all the paths / type etc as well (or just use the copy and paste commands)
You can use the file menu and export for screens command to generate files from all the boards. Select them in the dialog and save as PNG etc but you can also save them as PDF as single file or multiple files (do that via the format line). You can also use the export as and save them as PSD / PNG etc. You can also use the save as command and save as PDF etc
You can remove them by selecting them and pressing the delete key or click the little dustbin on the control bar to delete them.
You can use this feature to cut up text or anything. Open a file and then go to the tool and create a board anywhere over the design, in multiple places, overlapping, perhaps create multiple copies of the original image and rotate them or add effects to them etc and again add more and more entries across the entire document. You can then use export feature and instead of one file, you will have tens of sliced and diced parts of the original document.