I am from the United States and "low key" is a commonly used slang term. You would use it if you wish to share something with someone that you would like to be discreet or secretive about. You could also use it if you don't want to draw attention to yourself or others. Your example is a correct use of the expression.
More examples include:
- They're dating, but it's nothing serious. They are trying to keep it low key.
- I want to go out tonight but want to keep it low key. Let's not get too crazy at the party.
- If you are going to go out, can you be low key about it? I don't want everyone to know.
- They arrived late for work but were very low key about it.
- My weekend was pretty low key because I didn't do anything special.
- If you are planning on telling her, please be low key about it. I don't want her to get upset.