It's basically taking a small out of context word or phrase and making it seem like the person's entire argument based upon that phrase. Like the proverbial strawman, it's a hollow representation.

For instance let's say we're having an argument over climate change. I say "the data says that in 20 years this planet will be unrecognizably changed by the climate shift if we don't implement policy changes now." If I were to debate that with the intent to make a strawman I'd argue "my opponent is saying the world as we know it will end in 20 years."

While not technically false, I did say the world would be unrecognizable, it misreprents the actual point I made in order to make it easier to oppose.