It is hard to judge the size but it looks like an umbrella stand. Is the bottom metal? If so and since the staining is on the inside where it is hidden I wouldn't bother to try and clean it. The purpose is to store dripping umbrellas and staining will be inevitable unless you line it with something.

I agree with floral __ uk. This looks like an umbrella stand. It’s very interesting looking and if you’d like to pursue cleaning ideas, I’d do a search on YouTube. It looks like it may have a crack so I don’t know how to handle that with any cleaning products.

The bottom does not appear to be metal. Seems like it’s ceramic throughout. But, I could be wrong. 😅

If it is an umbrella stand and was used for that, it's possible the metal tips sitting in the water dripping off the umbrella cause the rust/iron deposits. You can try something like CLR to remove them, but if there is no metal in the bottom the stains won't hurt anything. If there was metal and it was oxidizing, it could cause damage, but not to ceramic.