This mark is identified as a plane control marking and based on an apron surface. What does it mean?
It's a stop sign. Implying stop, then yield.
Here, a left-right access road crosses an up-down taxiway.
The sign directs street vehicles, not airplanes.
In US/DOT vernacular, widely copied: shape is everything. All octagonal signs are stop signs, just as all inverted triangle signs are yields, all round signs are RR approaches, and all crossbucks are RR tracks. They can be snow covered, they can have a clarifying suffix such as "Eating Meat", they can be paisley, they can be green and say "Go". They all mean "stop, then yield". The shape overrides the art.
Obviously, this is painted on a surface, rather than on a standing sign, for operational reasons. But the choice of octagon, coupled with red with white border design language, does everything possible to invoke the standard DOT style of a highway traffic stop sign. It would be grossly negligent for any surface vehicle operator to do anything other than stop and search for something to yield to.
Note that a standard STOP sign gives no clue as to what exactly you are yielding to. The driver is obliged to figure that out from context. (Although sometimes on the highway, DOTs will add nearby clues outside of the stop sign proper; here there would be a supplemental sign indicating "taxiway" in words or graphics).
This particular STOP sign is non-standard, and no DOT would install it on a highway. However the design language makes the purpose crystal clear: Stop, then yield for something... With a big hint as to the typical "something".
And the design is a bit whimsical; obviously the yellow line does not separate two distinct lanes of airplanes as the graphic might suggest.