It's a small device that keeps your keys offline and signs transactions without exposing them to your phone or computer. It's the usual next step once your savings grow.
Go deeperPopular options include Coldcard, Trezor, BitBox, Foundation Passport, Blockstream Jade, and Ledger. Look for open-source firmware, a good track record, and Bitcoin-only versions if you only hold bitcoin. Always buy straight from the maker, never a used one or from a third-party seller.