This means to talk about something briefly. Even if a person says only a few words about a topic, that would be considered "mentioning" that topic. Other words like "say", "talk about", etc. sound like they require a longer period of discussion.

You can use "mention" with a simple noun:

Did she mention me in the article?

Or you can use a dependent clause:

Did he mention where he was going?

You can also use the structure "mention (doing something)"

He mentioned going to the gym earlier in the day.