i'm new to the cycling scene and am trying to maximize all my energy on the right gears.

I "understand" everyone when they are referring 50/34 t to 12-27 cassette...but I don't really understand the actual mechanics.

Can someone explain this to me...

lowest gear would be 34/27; highest gear 50/12?

how big of a difference will I see in the cassette when it's 11-25 to 12-26? (# just as ref)

I'm trying to find right crank to cassette ratio for me