da-DUM - as in "but look!" or "You see"
Iambic pentameter therefore means five iambs in a row:
da-DUM da-DUM da-DUM da-DUM da-DUM
If MU-sic BE the FOOD of LOVE play ON.
Now IS the WIN-ter OF our DIS-con-TENT
But SOFT what LIGHT through YON-der WIN-dow BREAKS
It's a lovely meter to use, as it creates a lilting flowing style, longer than conventional poetry but shorter than free-flow prose. The iambic flow helps to carry the speech along (especially helpful during somewhat long-ish speeches), and five beats per line gives it shape and form.
In Shakespeare's work the strict iambic form is occasionally deviated from, for example the famous "to BE or NOT to BE that IS the QUES-tion" ends in a weak stress known as a feminine ending (and even then the stresses don't quite work, the mind naturally wants to say "to BE or NOT to be THAT is the QUES-tion). However the meter still flows, and during the speeches, or suitably poetic discussions between high-class characters, the flow is all. Free prose (with no stresses, so just said as people normally speak) is reserved exclusively for lower-class characters during humorous interludes, such as the drunken porter in MacBeth. The sudden lack of poetic meter signals to the audience that this part is not relevant to the plot so much as an excuse for bawdy or topical jokes and probably a fair bit of slapstick.
A question often asked is how much the meter should be stressed during performances, obviously you don't want an actor who overstresses every second syllable, but performances where the meter is ignored completely and the whole thing spoken as prose always seem to me to be lacking something. The best performances are usually by those actors who can speak using the iambic stresses but make it sound like that's how they normally speak. The wonderful soaring flow of the meter makes grand speeches grander, desire more beautiful, pledges of love more real and meaningful, and pretentious idiots who think their in love sound even more deluded and pompously grandiose than normal "If music be the food of love!"