What does the orange exclamation mark mean in Toyota Highlander?

One or more of your tires have low air pressure and that is including the spare tire so check that one also

Added: Even after you inflate them all up to proper pressure the warning light will not automatically go out, you must find the reset button (located at the bottom edge of the dash just to the left of the steering column, and depress and hold for 10 seconds. The instructions are in your owners manual.
As mentioned by the first contributor, it is USUALLY the spare tire which is the culprit.