On the same wavelength means understanding each other without having to explain. We are on the same wavelength.
The picture is from radio, where two sets must be tuned alike to hear anything. Being on the same wavelength is about how people think rather than what they agree about.
1phraseon the same wavelengthUnderstanding each other without having to explain.
“They had been on the same wavelength since the first meeting.”
on the same wavelength is a phrase.
Practised in Idioms