A slash character (/), normally used for self-closing tags, has been found in an unexpected place for a tag, and the validator has decided to ignore it and treat is as a start tag instead.
HTML tags can go in pairs (start tag, end tag) or be self-closing, for example:
This is a paragraph with a start and end p tag
In HTML5, the ending slash and the preceding space is optional for self-closing tags so all of these are valid:
A common mistake is including a slash tag on a start tag, as in the following example. If the Nu HTML checker encounters that, it will ignore the slash and just treat the tag as a start tag.