I dont really understand what offset really is. I did some info searching on the internet and im sure it has something to do with the size of the rim and whether it will rub against the wheel well or the shocks.

Can someone please help me better understand what an offset is. Im planning to buy some 18"

Check here:

http://www.tirerack.com/wheels/tech/offset.htm

http://www.scionlife.com/forums/viewtopic.php?t=348

If offset is high (40+), then your tires/wheels will be installed more towards inside. Lower the offset, outside tires/wheels installed. SCION's OEM wheels have 38+ offset. If you look your car from behind, you know what it means. Tires are located inwards and making SCION somewhat look unbalanced. This is why people are looking for smaller offset wheels and even adding spacers.