Behind closed doors means in secret; away from observers, reporters, or intruders, usually in a closed room.

For example:

You can’t tell anyone what happens behind closed doors here—it’s classified information.

What I do behind closed doors is none of your business!

Sure, they seem happy together, but who knows

what happens behind closed doors.

The negotiations were held behind closed doors.

Many decisions were taken in secret behind closed doors.

Tradução:

AI Overview

“Behind closed doors” significa “a portas fechadas”, “em particular” ou “em segredo”. Refere-se a algo que acontece de forma confidencial, sem a presença ou conhecimento do público em geral. 

• The negotiations were held behind closed doors.” (As negociações foram realizadas a portas fechadas.) 
• “The committee usually meets behind closed doors.”: (A comissão geralmente se reúne em segredo.)