Flask comes packaged with the powerful Jinja templating language.

For those who have not been exposed to a templating language before, such languages essentially contain variables as well as some programming logic, which when evaluated (or rendered into HTML) are replaced with actual values. The variables and/or logic are placed between tags or delimiters. For example, Jinja templates use {% ... %} for expressions or logic (like for loops), while {{ ... }} are used for outputting the results of an expression or a variable to the end user. The latter tag, when rendered, is replaced with a value or values, and are seen by the end user.