Offset & Bolt Circle
The offset of a wheel determines how far in or out it will sit in relation to the hub and also your vehicle's guard / bodywork. Each vehicle has an optimum offset range and the further you deviate from it, the higher the chances are of the wheel not fitting properly.
When compared to the offset of your vehicle's factory OEM wheels, an aftermarket wheel with a negative (higher) offset will stick out further from the side of the car, potentially allowing it to hit the guards whilst cornering or going over bumps. Alternatively, an aftermarket wheel with a positive (lower) offset will sit further inwards from the side of the car, potentially causing it to hit or rub on suspension components or the inner walls of the wheel well.
When upgrading your vehicle's factory OEM wheels, it is also vital that your new wheels feature the exact same bolt circle measurement. You might have seen bolt circles of 5x114.3 or 6x139.7 - what these list are the amount of wheel studs and the diameter of the circle they're arranged on.
So a 5x114.3 wheel has five wheel studs arranged along an imaginary 114.3mm diameter circle around the centre of the wheel.