Can someone tell me why arm is so good? I don't understand the difference.
The biggest advantage is lower power consumption. Both in the CPU core and the memory subsystem because ARM requires less memory bandwidth for a given level of performance than an architecture like x86 does. This is largely down to the way that ARM uses a load-store architecture where most operations are between internal CPU registers and x86 has an old-school CISC architecture where many instructions operate between registers and memory, which obviously involves memory access cycles.
The x86 also has a lot more overhead because it's processing CISC type instructions but executing them in heavily pipelined RISC type execution units - this basically forces the CPU to decompose the instructions into simpler elements, resolve any scheduling conflicts and then dispatch them for execution. This is being done at run-time inside the CPU. The equivalent steps on a RISC CPU are carried out once, by the compiler, which obviously doesn't have any effect on run-time power consumption.
This is especially significant for Apple because they are heavily focused on making their machines very small and thin - which means that often the ultimate limit on the available performance is imposed by cooling - so a chip that uses less energy will enable more performance in the same chassis, along with longer battery life.
