I know the symbol is for tire pressure monitor, but what does
"MAIN" and "2nd" mean, and it's purpose?

Then of course the curious symbol of the left and right arrow pointing to the center dot.

I have had this truck for a month and just discovered it just right of the base of the steering column:



Also what happens when you press in the button for "2nd"?

 

I think it's to allow you to switch the TPMS between two different set of tires.

 

Makes sense to me. Like perhaps winter and summer tires?

 

That's right.

 

I never even noticed this button! LOL, but thanks for pointing it out to me!

 

I never noticed too, untill I was rotating my tires.