What is an invoice?
An invoice is a detailed bill that lists what was bought, how much each thing costs, and the total amount still owed. It always shows key dates, like the invoice date and the due date, so both people know when payment should happen.
For example, if John Hotdogs sells three hotdogs to Clara for $5 each on March 3, he sends her an invoice that lists March 3 as the invoice date, March 10 as the due date, and the total $15 she still owes. Keeping those dates clear helps Clara pay on time and helps John track his earnings.