HELP Rim measurement explanation please

[mattypsd]

I currently have 20x9 6.25 back space.. with positve 30 mm offset
my 12.5 inch wide superswamper radials.. rub when i turn the wheel.

i want to run a 38. x 15.5 ( toyo which they say is actually 16.5 inch wide)
and i need help on what rim to get so it doesnt rub

if someone could explain how offset and back spacing works i woudl appreciate.

i know neg offset pushes the rim out thats all i know..