With Scale up, the resources you run on can only be increased up to the limit of the existing hardware, and the cost for those resources doesn't increase in a non-linear way.
Scale out means that, instead of increasing a machine's capabilities, you add more machines. With Scale out (also known as Horizontal Scaling), you can scale your application indefinably, and then pay in a linearly–multipling the number of machines by the price per machine.
Scaling out allows you to distribute the processing of requests between several machines dynamically. When there is a high load, you can add machines, and when the load decreases, you can decrease the amount of machines you use. This way, you pay for your actual usage ...
