A wheel is described as something like 18 x 8J ET45. The 18 is the diameter in inches, the 8 is the width, the J refers to the flange profile, and ET45 is the offset in millimetres.
Offset is the distance between the wheel's mounting face and its centreline. A positive offset pushes the wheel inward toward the suspension; a lower or negative offset pushes it outward toward the arch.
Change the offset too far and you invite problems: rubbing on suspension components or arch liners, altered steering feel, and extra load on wheel bearings. A few millimetres is usually tolerable, but treat the manufacturer's figure as the target.
Also check centre bore. If the wheel's bore is larger than the hub, it needs a spigot ring to sit concentrically, otherwise you will feel vibration no amount of balancing fixes.