What do these maps display?

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The amount of times a state has voted for the respective party going back to 1976.

Judging by the fact that Minnesota is in the lightest blue while the plains states are in the lightest red, I think those maps use shading to indicate how many times a state has voted for a certain party's Presidential nominee in the last X elections.

You are both very close, but it's not quite correct.

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This sequence could not have started before 1976, because Mississippi, Alabama, South Carolina and Texas, which voted Democratic last time in 1976 have the same color than Oklahoma, Kansas, Nebraska, Utah, Idaho and Wyoming, which voted Democratic last time in 1964. If the sequence started in 1964, Mississipi, Alabama, South Carolina and Texas would be darker than Oklahoma, Kansas, Nebraska, Utah, Idaho and Wyoming.

I think it means last time a state voted for the respective party since 1980. Darkest = 2012, lightest = no vote

What?
You'd better buy a new monitor or new glasses, respectively...

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last time a state voted for the respective party. darkest = 2012, lightest = 1964/1972, respectively.

Last time a state voted for a respective party.

Congrats! You got it right.