An iambic line is organized in units of one unstressed and one stressed syllable. Here is an example for Shakespeare's "Sonnet No. 75":
"That time of year thou mayst in me behold."
In this line, the syllables "time," "year," "mayst," "me" and the "hold" part of "behold" are all stressed (or emphasized), while the other syllables are less emphasized. The words "that time" make up one iambic foot, meaning one unit of unstressed and stressed syllables. Iambs are two-syllable meters. Iambic meter is the most common natural meter in the English language.
Trochaic meter is the opposite of iambic, featuring one stressed and one unstressed syllable, as demonstrated below in a line from "A Psalm of Life" by Henry Wadsworth Longfellow:
"Tell me not in mournful numbers."
In this line, the syllables "tell," "not," "mourn" and the "num" part of the world "number" are all emphasized, while the others are less emphasized. The trochee is a two-syllable meter.
An anapestic foot is made up of two unstressed syllables followed by one stressed syllable, making it a three-syllable meter. This line from "Break, break, break" by Alfred Lord Tennyson demonstrates the Anapestic line:
"And the sound of a voice that is still."
Here, the syllables "and" and "the" are both unstressed, while "sound" is stressed. The next two feet follow this pattern.
The dactyl, like the anapest, is a three-syllable foot, though it inverts the anapestic pattern by starting with a stressed line followed by two unstressed syllables, as demonstrated in this line from Henry Wadsworth Longfellow's poem "Evangeline":
"This is the forest primeval, the murmuring pines and the hemlock."
In the first foot, the word "this" is emphasized while the syllables "is" and "the" are unstressed.