A self-custodial or non-custodial wallet is a digital wallet that is controlled by only the people who know its private key/ seed phrase.
Every self-custodial wallet will give you a 12-word/ 14-word phrase, which act like the "password" to your wallet. When you input these words to that wallet, you gain access to your assets.
On the other hand, custodial wallets don't employ the concept of seed phrases. They have full access to your wallet. These wallets are central exchange accounts like Coinbase, Gemini and Binance.