Clean dry coarseish building sand. That tub it's in is about 45cm diameter x 33cm high, so basically a barrow load - about 60 litres or 90 kg.
It's been used to fill a plastic Ikea 4-piece parasol base weight which holds down a cross-shaped parasol base.
The Ikea item is discontinued, I think, but it's this:
https://www.ikeaddict.com/ikeapedia/en/Product/10285328/gb-en/
Take either or both, but prefer to give both together. If you want the sand, bring some bags, because that tub is too heavy for any normal human to lift.