Hopefully "semantically meaningful" is pretty self-explanatory, it just means using HTML elements in an organized and structured way (like not using a span when you should use a paragraph). "Accessible" means usable by screen readers and potentially other accessibility tools for people with low vision or other disabilities.

As an example: Consider using a <div class=“paragraph”> vs using <p>. The <p> tag is semantic and meaningful, whereas the <div> means nothing. A screen reader (or other markup interpreters) can’t tell what a <div> means