20 by 10 wheels

im looking to get 20 by 10in wheels and it say some thing about a 24mm offset. sorry for the dumb ? but what the hell is that i have no clue what that is and will i need any backspaceing. can some one help me out thanks guys for all the help.

Here are a couple of pics to give you an idea of what it means. It helps if you put the vehicle you have in your question H1, H2, H3.
 
Here is the H3 Specifications and Limits:
 
OEM tires:  265/75-16 (802x265mm). Big Hub. 32 x 10.5 x 16
Bolt OEM tires:  265/75-16 (802x265mm). 
Bolt pattern: 6x139.7 (6x5.5).
Offset: 15 to 35 mm.
Tires up to 842x305mm.
Rims up to 26x10.0".
265/75R 16 = 32 X 10.50 X 16
265/70R 17 = 32 X 11.50 X 17
275/70R 17 = 32.50 X 11.00 X 17
285/60R 18 = 32 X 11 X 18

its an h2 my bad i still dont understand all of this im a little slow can you still help. here this is what i want to put on
Size: 20 x 10
Bolt Circle: 8x6.50
Offset: -24mm

here's a page that explains offset.  usually easier to read the description than get partial info from us (well, especially me, cuz i leave it up to the guys at the tire store and it's worked everytime so far) . . . . .
 
http://www.nice-wheels.co.uk/pcdandoffset.html