A cryptocurrency wallet is a piece of software that digitally stores the private keys necessary to sign bitcoin transactions on distributed ledgers. They're an important part of the cryptocurrency ecosystem since they're the only means to prove ownership of digital assets and conduct transactions that transfer or change them.
They're also referred to as "crypto wallets," and they work similarly to the keys to a blockchain car. The car will not start without those keys. Without them, it would be impossible to verify ownership of a digital asset, which could be anything from a bitcoin to a token representing some type of asset.