What exactly does kernel_task do? I'd just like to know what it is that kernel_task is responsible for. Also, what's an acceptable maximum RAM and CPU use for it?
The kernel task is special since it's an entire operating system / micro kernel based on Mach 3.0. It abstracts most of the core hardware, timing, message passing and memory handling. 

What most people notice in Activity Monitor is that kernel_task accumulates all the CPU time for input/output (i/o) processing delays and calculations as well as scheduling overhead of processes and threads. Similarly - it accumulates all memory allocations that are not in the user space. On the 2014 and newer Macs, this task exists to throttle the CPU - when the temperatures in the processor are too warm, kernel task is assigned "no work" and shows high CPU busy to let the processor idle and not overheat the computer. This is most noticeable on the portable line - especially the MacBook without any sort of fan or blower to cool the processor. Apple warns of this to potential kernel programmers in "Why you should stay away from programming in the Kernel" that disabling cooling will cause permanent irreparable hardware damage.


  To summarize, kernel programming is an immense responsibility. You must be exceptionally careful to ensure that your code does not cause the system to crash, does not provide any unauthorized user access to someone else’s files or memory, does not introduce remote or local root exploits, and does not cause inadvertent data loss or corruption.


If you watch it after a reboot you will see it starts with minimal RAM and CPU needs and instead grows only when other programs run and need the system services. 

To slim it down, shut off unneeded ports (WiFi, bluetooth, etc... ) as well as idle / unused programs. If things don't settle down, consider rebooting to ensure no task is stuck or leaking memory.  If you have programs doing real work, they will cause the kernel to use both memory and processor time - just watch as you start things to see which other programs are causing this behavior. It's rarely an issue with the kernel - even with badly performing programs - that can't be solved by logging out and back in again. The kernel_task cleans up well and generally takes care to only use the resources needed by the workload.