* You tried to include the "<" character in your page: you should escape it as "<"
* You used an unescaped ampersand "&": this may be valid in some contexts, but it is recommended to use "&", which is always safe.
* Another possibility is that you forgot to close quotes in a previous tag.
Most likely, you nested tags and closed them in the wrong order. For example <p><em>...</p> is not acceptable, as <em> must be closed before <p>. Acceptable nesting is: <p><em>...</em></p>
Another possibility is that you used an element which requires a child element that you did not include. Hence the parent element is "not finished", not complete. For instance, in HTML the <head> element must contain a <title> child element, lists require appropriate list items (<ul> and <ol> require <li>; <dl> requires <dt> and <dd>), and so on.