A user sandbox is a personal wiki page(s) where you can experiment, practice editing, plan out articles, or begin drafting articles before moving them into the article "mainspace” on Wikipedia—where live articles are read and edited.
To go to your default sandbox page, simply click the Sandbox link, which can be found at the top right whenever you are logged in.
Open up your sandbox and try writing something. Anything. This is a place to experiment. Play around.
You can try making links to Wikipedia articles, adding bold and italic text, dividing the page into sections using headers, and creating footnotes.
Don’t forget to click on "Save page” when you're done editing.