Shakespeare's famous for blank verse in his plays, and Henry VI, Part 1 falls right in line. Blank verse has no rhyme, instead featuring iambic pentameter, which has ten syllables per line, and follows a pattern that roughly emphasizes every second syllable. Check out this example from the play—we've added italics to show the stresses—when Gloucester is praising Henry V, and he says:
His brandished sword did blind men with his beams; His arms spread wider than a dragon's wings; (1.1.10-11)
But Shakespeare's also famous for breaking the rules to make a point, and to keep things interesting. Can you imagine if this entire play followed the same rhythmic pattern? No thank you. So from time to time Shakespeare does something else, like throwing in a rhyme. When this happens, chalk it up to Shakespeare's poetic license—but also pay attention. Sometimes when patterns are broken, the writer is trying to get our attention.