A demand deposit is a document that allows you to receive money from someone else’s bank account. The most common type of demand deposit is called a check.
The bank will issue you a set of paper documents with your bank account number preprinted on them. If you fill in the name, say John Jones, fill in the amount, say $100, and sign it, the bank will take $100 out of your account and give it to John Jones. The check John Jones has is a demand deposit because it allows him to demand money from your bank account.