I mean... computers are complex these days. I could type up like 3 paragraphs that more specifically describes what is going on but is that really helpful?

Yeah, pipelines and out-of-order exeuction makes the definition a bit difficult. If you want to ensure that all previous instructions are done executing, you need lfence, and if you want to prevent future instructions from filling in the pipelines you'll need an mfence.

There are many clocks (even within a core). The turbo-clock is different from the standard clock. I forget exactly which clock rdtsc uses, but I do know that under some processors under certain conditions, you'll get weird results.

Different processors may have different interpretations of "clock" (mostly due to turbo and/or sleeping behavior). Etc. etc. I don't recall the details, but these different clock states could vary as much as 2.2GHz to 4GHz on my processor (P1? Turbo? I forget the exact name...)

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But all in all, you get a 64-bit number that describes the number of clock-ticks --- for some "definition" of clock tick that differs between processors... and for some definition of "now" (in the case of out-of-order execution and/or pipelined execution, the "now" is a bit ambiguous, as previous instructions may have not finished executing yet and future instructions may already be executing).

If you really want to know, read the processor manual specific to the microarchitecture (since different microarchitectures could change these definitions)