What are portmanteau words?

From Lewis Carroll’s Through the Looking Glass:

‘You seem very clever at explaining words, Sir,’ said Alice. ‘Would you kindly tell me the meaning of the poem called “Jabberwocky”?’

‘Let’s hear it,’ said Humpty Dumpty. ‘I can explain all the poems that were ever invented — and a good many that haven’t been invented just yet.’

This sounded very hopeful, so Alice repeated the first verse:

‘Twas brillig, and the slithy toves
Did gyre and gimble in the wabe;
All mimsy were the borogoves,
And the mome raths outgrabe.

‘That’s enough to begin with,’ Humpty Dumpty interrupted: ‘there are plenty of hard words there. “Brillig” means four o’clock in the afternoon — the time when you begin broiling things for dinner.’

‘That’ll do very well,’ said Alice: and “slithy”?’

‘Well, “slithy” means “lithe and slimy.” “Lithe” is the same as “active.” You see it’s like a portmanteau — there are two meanings packed up into one word.’