What does the BASE clock speed of an Intel CPU really signify, now that we have Turbo Boost and SpeedStep

The base clock speed is a guaranteed clock speed for full typical (but not peak) utilization. As long as one is not running a power/thermal virus (i.e., not peak utilization), one should be able to fully use the hardware at the stated clock speed without violating the thermal design power.

When software is not using all the available cores, or sufficient cooling is provided, some additional thermal headroom is available which can be exploited by increasing the clock speed of the active cores (Turbo Boost).

Incidentally, minimum clock speed is not especially useful for evaluating energy efficiency since a significant amount of power consumption is static power (independent of the work done). At a certain point, race to sleep (a.k.a., hurry up and go to sleep) will be more energy efficient than reducing the clock speed because sleep states reduce the static power use.

The Thermal Design Power (TDP) of Intel cpus is also tied to the base clock speed of the processor.
So when the processor is running at the base clock on all cores it should not exceed the TDP, but if it uses Turbo Boost the power consumption of the cpu can be much higher than its TDP.
So in use cases where the processor is tied to its TDP as the power budget for e.g. in some notebooks or some OEM machines the cpu will most likely not exceed the base clock when its load is at 100%.