A licence to print money
Definition 1 of 2
Pro Tip 1/3Colloquial Usage
This phrase is casual and mainly used in informal conversations or in the media for impact. 
When he invented that app, it was a licence to print money.
Pro Tip 2/3Metaphor
It's a metaphor suggesting the ease of making money as if one could literally print it. 
Her viral product turned out to be a licence to print money.
Pro Tip 3/3Idiomatic Expression
Understand that it's an idiom, not to be taken literally, but to emphasize profitability. 
That franchise is a licence to print money with its low costs and high demand.