A modal window is a panel that appears over a page and asks someone to deal with it before returning to the rest of the page. It is like pausing a conversation to handle one important question.
For example, a website might ask “Discard your changes?” before closing an unfinished form. The person can choose whether to continue or go back.
Use a modal for a decision that needs attention, not for ordinary extra details. Make its choices clear and provide an obvious way to close or cancel when that is safe.