On x86, there are actually 4 ring levels (0 to 3), but only two (0 and 3) are used for everything. On modern hardware there are also virtualization and service, and remote management rings, sometimes referred as -1, -2 and -3.

what is that stopping us(assuming we want to harm system of course) from damaging our system

Some CPU instructions only work at a certain ring level or lower. For example, changing memory mappings, can only be done from ring 0 or below, so a userland program running in ring 3 that would try to access some other programs memory, will get an "forbidden instruction" exception, that would escalate to the kernel's handler, and it could decide to kill the "malicious program". There are also many interrupts a ring 0 program/kernel can set, to intercept different program behaviors and handle them as it sees (allow, modify, redirect, block, log, etc.).

In order to "harm your system", as in wreak havoc with other programs, you need to either use a kernel function in some way, or get your code to execute at ring 0 (privilege escalation).

If you mean "harm your system" as in actual hardware, some drivers might allow you to overclock something, turn fans off, and end up with your GPU melting... but that would be a protection failure from the driver/hardware (hardware itself can have anti-overheat protections).