Pay a Debt of Honor

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Sep 12, 2017 13:24
Could you tell me the meaning of "pay a debt of honor?"

This morning, I found two tweets.

Vice President Pence @VP
.@POTUS asked Karen & me to be in Shanksville today to pay a debt of honor to the heroes of Flight 93 & their families. Never forget 9-11.

Karen Pence @SecondLady
Traveled to Stoystown, PA to pay a debt of honor & gratitude to the courageous men & women on Flight 93. I will always remember.

I couldn't understand the phrase of "pay a debt of honor", so could you tell me what that means?
(できれば相応しい日本語訳も知りたいです。)