A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Normally when you bring in slides from another presentation by copy/pasting or the Re-use Slides feature, PPT asks (sometimes a bit obscurely) whether you want to preserve the formatting of the original slide or apply the formatting provided by the template of the presentation you're adding the slides TO.
If you choose to preserve the original formatting, that'll include any font choices from the original, so your presentation will need fonts that may not be part of the current template. If the needed fonts are a) not installed on the current computer or b) not embeddable fonts, you'd get the message you're describing.