What does it mean when a recipe says "cut in butter"?

I'm making an apple crumb topped pie and for the topping i need to combine brown sugar, flour and then CUT IN BUTTER. How do i do this? i always used the recipe, but do the topping how i think it should be done, today i am taking to pie with us for after dinner and want it to be perfect... How do i do this??? show more I'm making an apple crumb topped pie and for the topping i need to combine brown sugar, flour and then CUT IN BUTTER. How do i do this? i always used the recipe, but do the topping how i think it should be done, today i am taking to pie with us for after dinner and want it to be perfect...

How do i do this???
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