that is to say
US /ðæt ɪz tə seɪ/
UK /ðæt ɪz tə seɪ/

1.
used to introduce a clarification or explanation of a preceding statement
:
•
He is a polyglot, that is to say, he speaks many languages.
•
The event is scheduled for next Tuesday, that is to say, October 26th.