I don’t know what smart thermostat to use with my high voltage
The question here is:
Can someone help explain what the wiring means?
This looks like a Barber-Colman TC-115. I couldn't find any circuit diagrams for it, but simple thermostats like what I think you have typically work like switches. When heat is needed, it shorts two wires together (heat and common). When cooling is needed, it shorts the other one with the common (cool and common). You'd need the circuit diagram and to test it, but it's possible red is heat, blue is cool, and yellow is common (again, you must check this with a multimeter and/or circuit diagram to confirm!)
Also, thermostats can take your main AC voltage, or they can take a stepped down voltage (24V AC, for example), or they could use DC for signaling.
EDIT: this is for informational use only about what the wires do. If you aren't experienced with this or have the right tools, it is dangerous. As far as I know, high voltage AC thermostats require special smart thermostats. I think the most common big box store ones can't handle it.