Pentameter is a line of poetry with five beats. "Penta" means five. Another way to describe pentameter is a line of poetry with 10 syllables. There are different types of pentameter. The most common one in English is iambic pentameter, which is a line of 10 syllables with alternating unstressed and then stressed.
From King Lear-example of pentameter that is not iambic:
Why should a dog, a horse, a rat have life,
And thou no breat at all? Thou'lt come no more,
Never, never, never, never, never!
Pray you undo this button: thank you, sir.
Do you see this? Look on her, look, her lips.
From Romeo and Juliet-example of iambic pentameter:
But soft! What light through yonder window breaks?
It is the east and Juliet the sun.
Arise fair sun and kill the envious moon,
Who is already sick and pale with grief,
That thou her maid art far more fair than she.
From Sonnet 18 by Shakespeare:
Shall I compare thee to a summer's day?
Thou art more lovely and more temperate.
Rough winds do shake the darling buds of May.