The HTML readonly attribute is a boolean attribute used in form input elements to prevent modification of the element's value by the user. When applied, it makes the field non-editable, meaning the content displayed in the input field cannot be changed directly through the user interface. However, users can still select and copy the text from a readonly field, and the field's value will be sent when the form is submitted. This attribute is particularly useful for displaying pre-populated values that should not be altered by the user, such as a confirmation number or a fixed amount in a payment form.

It's important to note that readonly can be applied to various types of input elements, including text, date, datetime-local, email, month, number, password, search, tel, textarea, time, url, and week. Unlike the disabled attribute, which also prevents user interaction, readonly fields remain focusable and can receive focus, which means they are still accessible to keyboard users and screen readers. Additionally, while disabled fields are not included in the form submission, readonly fields are included, allowing their values to be collected and processed on the server side.