I'm not sure this is an idiom, but it's a little phrase that you hear over and over again. It's used as a tag at the end of a sentence when you want to say 'isn't that so?' In English, this can often be expressed as one of a number of tags, like,'don't you, haven't you, won't you, etc. So you might hear:
'Quieres ir al café, ¿a que sí?' which would be 'you want to go to the café, don´t you?'
As an answer to a statement, it can have the flavour of 'absolutely', or 'you bet'. For example:
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