Does "description" in Schema.org markup have to be free of HTML tags?

If you are using the newer JSON-LD format for schema data then it seems (at least with Google) that it's OK to include HTML markup for some fields - indeed, they actually ask for it. See their recommendation for JobPostings, for example, where they state for description:

The full description of the job in HTML format.

You must format the description in HTML.

Their Structured Data Testing Tool certainly doesn't seem to complain about HTML in fields, though you probably want to limit it to basic tags. Whether other parsers are OK with it is difficult to say.

You are using the meta element. Its content attribute can only contain a string. If you’d provide a value like <b>bold</b>, <b> and </b> would be part of the value, and these would be interpreted as text, not as markup.

For Schema.org’s description property, you can of course use a different element (like p). This may contain markup, but the value for the description property will be the text value.

So for

<p itemprop="description">foo <b>bar</b></p>

the value would be "foo bar".