"Sample Accurate Clock" is a term that describes the use of audio pulses that provide a much higher degree of reliability and precision when being sent from a Digital Audio Workstation (DAW) instead of MIDI Clock.
Audio tracks can't drift apart. When square waves are placed on a dedicated audio track and aligned to the grid of a DAW, this creates a clock source that is locked into the audio sample rate during playback.
There are devices that can receive these audio pulses and translate them into other clock protocols such as MIDI Clock and/or CV Sync Triggers. One such device is CLOCKstep:MULTI. The end result is a clock signal between your DAW and other external gear that is in lockstep.
Contrast that to MIDI Clock coming from a DAW. MIDI Clock is not audio, it's packets of serial data that move over DIN or USB ports. There's no low-level linkage between the audio that's being played on the computer and MIDI Clock, so there's nothing that will prevent the MIDI data stream from either drifting away from the audio stream or introducing jitter, especially on a multi-tasking computer when its resources are taxed.
The idea for "Sample Accurate Clock" actually comes from a time before the digital revolution fully took hold in music. For a time, studios were a mix of Analog Tape Recorders and Digital Sequencers. To keep these in sync, a track on the Analog Tape Machine was striped with audio information representing time. A digital machine could use this information to stay in sync. No matter what mechanical variables were present on the Tape Machine (ie: wobble and speed), every track on the tape would still pass over the tape heads at the same time, including the track containing the timing information. Simple and very effective.
Here's a video of CLOCKstep:MULTI being demonstrated using Sample Accurate Clock with Ableton Live 11, and another video showing how it could even be used with Analog Tape like the days of old.