Running WRX Blob & looking to change my alloys but can someone explain what 'offset' means & what I need to purchase as I don't want to end up with the wrong alloys.

 

The offset is the distance from the mounting face of the wheel to its centre. So, a wheel with a low offset eg ET34 will be 20mm further out toward the wheelarch than an offset of ET54
For instance, my PFF7 wheels have an offset of ET51 (IIRC) Using a wheel with a very low offset may look good but may impose extra loading on the wheel bearings. Choice of offset is often governed by the clearance of the caliper to spokes.

JohnD