it has five feet, each foot consisting of one unstressed (or unaccented) syllable followed by one stressed (or accented) syllable. This type of foot, with one unstressed syllable followed by one stressed, is called aniamb (this is where the adjective iambic comes from), and the word pentameter comes from the fact that there are five (penta-) feet per line.  

However, things get a little trickier after that first line. The second line seems to begin with two feet which do not follow the iambic pattern. They are called trochees, and a trochee has one stressed syllable followed by one unstressed. Often, trochees can sound more aggressive than iambs because the accent comes first rather than second in the foot, and it makes sense here that the poet would want to draw our attention to the terrible sounds of the soldiers' knees knocking together and their raspy and guttural coughing. Lines 3 and 4 resume regular iambic pentameter.

In line 5, another trochee is substituted for the third foot, disrupting the meter after the word "asleep" as if to jolt us, readers, out of the lulling effect of a regular meter. This substitution also draws attention to the word "Many" as if to emphasize the number. Line 6 begins with an iamb, but then another type of foot called a spondee is substituted for the second and third feet. A spondee consists of two accented syllables, and it tends to disrupt the rhythm, as they do here. Five accented syllables in a row slows down the pace of the line, and it is, I'm sure, no mistake that this occurs as the speaker is describing the soldiers limping, slowly and painfully. The rhythm here, in the two spondees, also becomes plodding and slow.

The next line begins with a trochee before resolving into the more regular iambs, and the final line of this stanza starts with three iambs, nice and regular, but then ends with an anapest, another kind of foot that consists of two unstressed syllables followed by one stressed. These two unstressed syllables occur just as the speaker describes the "soft" sound of gas shells dropping far away, and so the repetition of unstressed sounds seems almost to mimic the sounds they describe.

Ultimately, most of the lines in the poem are written in iambic pentameter (even though there are many substitutions, like the ones I've described). Clearly Owen uses accents to great effect, to draw our attention to certain words or descriptions and even to mimic their sounds.  

Owen changes the tempo or flow of the poem from stanza to stanza and within stanzas. The tempo of the first stanza is slow and plodding, reflecting the intense weariness of the soldiers. Owen achieves this slow effect by using long sentences, full of clauses, punctuated by commas that slow us down, such as follows:

Bent double, like old beggars under sacks, Knock-kneed, coughing like hags, we cursed through sludge, Till on the haunting flares we turned our backs, And towards our distant rest began to trudge

Gas! GAS!

Quick, boys!