ET is the offset, in mm, from the centre of wheel to the face where it fits the against hub.
So - if you currently have ET38 wheels, and you got some ET40s, they would sit 2mm further inwards. If you got some ET28s, they would sit 10mm further out.
With you current ET38 wheels, this means that 'positive offset' on the diagram below measures 38mm:
Hope that make sense?!
Surely that must drive weird though, with the power tailing off after 4700 rpm?