I used the symbols font on a slide created using PowerPoint for Mac, and when I opened the same file in Windows the characters turned into squares. Does anyone know what is going on?
You should avoid using the old Symbol font (where you type Latin letters and get symbols instead) in documents these days, as it is not Unicode and there is no way to guarantee it will be seen properly at the other end. Instead use the Character Viewer (Edit > Emoji & Symbols) to input your symbols from the Greek or Dingbats or various other symbol categories found there.
The Macintosh Symbol font dates from.. well.. the Macintosh. I used Font Book to do a "Get Info" on the Symbols .ttf font file on my otherwise up-to-date iMac, and the most recent copyright date it bore was 1999. Whatever charming memories may appertain thereto, I remember Symbol mostly as the one-time proprietary Adobe --> Apple PostScript font which Microsoft Explorer would reproduce on its web pages, but which Outlook would not reproduce in formatted email and MS Word would choke on in .rtf files. Before .pdf files made it possible to embed fonts and exchange WYSIWYG documents in a trivial manner, and before Microsoft eventually licensed Symbol from Adobe, we used to suffer along with (I think) Wingdings / Webdings substitutions for situations where the Windows folks had to have the material show up on their screens.
Thing is, PowerPoint is a world into itself--and only a limited set of fonts are reliable among Windows OS variations, much less when Macs are included. A web search for "safe powerpoint fonts" will show you that Symbol isn't among them--nor, as tubedogg pointed out, is it any longer part of a standard Mac system install.
Yes: The font you used is not installed on the target system, and whatever font it's using instead does not have glyphs for the characters specified.