As Bachi-Bouzouk says, you need to tell us what interface you're using. Vague questions require a lot of guessing on our part.
If you want to learn about Foobar, a good place to start is
here. Check out the title-formatting reference.
Foobar uses scripts to display song information. For example:
%artist% - %title%
shows:
Led Zeppelin - Stairway to Heaven
If the artist tag is empty or missing, Foobar will show a ?
? - Stairway to Heaven
This is not an error. It's not dangerous. It just means that Foobar has no information.
In your case, I would not worry about missing tags in wav files. Wav's are not designed to be tagged. Just convert it to whatever format you want and load those files into Foobar. Then worry about the tags.
If at that point you still get ? in your display, go to Preferences > Display.
If you're using the default interface, now go to Title Formatting.
If you're using the Columns UI interface, now go to Columns UI > Playlist View > Columns.
In either case, take a look at the formatting script. That will tell you what Foobar is trying to display. You should be able to figure out what is happening.
If not, post the relevent contents of your formatting script and we can help you from there.