A portmanteau is a large suitcase. The word comes from French porter "carry" and manteau "mantle, or cloak" — so it's what you carry your clothes in. Or, a portmanteau is a word made by combining two other words.
You might remember portmanteau from Lewis Carroll's Through the Looking Glass, where Humpty Dumpty explains to Alice the portmanteau word, in which "two meanings are packed up into one word." So, according to Humpty Dumpty, slithy means "lithe and slimy," and mimsy is "flimsy and miserable." You can make your own portmanteau words, and here are some examples for inspiration: smog (smoke + fog); brunch (breakfast + lunch), sitcom (situation + comedy), and infomercial (information + commercial).