Hello xue'er,
Tango,that South American ballroom dance is that sort of dance that requires 2 persons to be performed.
The saying refers to any activity that can't be carried out without the existence of 2 persons.
It is a verbal reaction in situations,where one person claims to be able to accomplish something,that can't be demonstrated or carried out by one individual and requires the assistance of one other person specifically.
The other person will use this saying as a reminder to the impossibility of performing this action solo.
It covers a wide range of activities,in fact any kind that requires 2 persons to be performed.
Example:
- Why don't you go and do your favorite sport?
- I love to play tennis,but can't find a company. I can't play tennis all by myself. It takes two to tango.
It could also mean, if two people were involved in a bad situation, both must be responsible.
"She blames Anna for stealing her husband. Well, it takes two to tango."
It means the wife is responsible as well for this bad situation.