When do we estimate in Scrum?

Hi guys,

I got a simple question: when do we (as a dev team) estimate in Scrum?

When I first learned the Scrum basics, back in 2014, I remember that it was said that the dev team was supposed to estimate during the Sprint Planning using technics like the Planning Poker or extreme Quotation.

But I am puzzled reading the Scrum Guide because it seems that estimation is recommended to be done during the Backlog Refinement activity.

Is that correct and if so what is the main reason why?

Thank you,

J.