The first person to ever use the phrase was Ronald Reagan in his 1980 bid for president...
Now, while I was certainly young in 1980, I see what Reagan did and see him as one of the greatest American presidents in U.S. history.
That said, the phrase reminds me of Reagan and all he did on the world stage. He was, likely first and foremost instrumental in facing down the communists on the world stage, and as such, instrumental in the defeat of the Soviet Union.
I see the same threats again today with Marxism etc. as we once had back then, except now instead of only having to face it down on the world stage, these mindsets have come home to roost right here... so the struggle is to regain economic American strength both on the world stage (defeating potential trade agreements like the TPA, TPP, TPT, the Paris accord and more that would have weakened us economically) but also fight against those Americans who believe we should have a purposefully weakened economic position, favoring other peoples and foreign nations above our own citizens.
So to me, make America great again is about putting American interests first (of all races), and standing against those who would defeat us from within. When the Soviet Union fell, the ideology was exported right here on our own soil, and we see the fight we have before us, for the ideology of our nation. To stand for American freedom, and American interests above all manner of communist (Marxist, Leninist, Stalinist, Socialist) thought.