Iambic pentameter is a common metrical form in dramatic verse. The meter is of five feet (iambs in the Greek) each made up of two syllables. The syllables are enunciated with alternating stresses (in English), generally the stress being on the second of each pair of syllables. So each line of verse sounds as follows: de-dum de-dum de-dum de-dum de-dum. There are of course lyrical variations.
You can easily spot Iambic Pentameter by inspecting and counting the syllables in this way.
Perhaps the most famous exemplar of the form is Shakespeare, who uses iambic pentameter extensively in his plays and sonnets. Consider the following famous lines from Romeo and Juliet, spoken by Romeo on seeing Juliet at her balcony window. Count the syllables in your head while reading each line and consider the rhythm of the form.
But, soft! what light through yonder window breaks? It is the east, and Juliet is 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: Be not her maid, since she is envious; Her vestal livery is but sick and green And none but fools do wear it; cast it off.