A monohybrid cross represents only a single character while dyhybrid cross represents two characters.

For example, if you cross pure-breeding tall pea plants with pure-breeding short pea plants, you are considering only 1 character: stem length.  That is a monohybrid cross.  If you cross pure-breeding tall, round-seed pea plants with pure-breeding short, wrinkled-seed pea plants, then you are considering 2 characters in the cross and it is a dihybrid cross.