Red Alert! This analysis refers to the poem in its original language. Translations may or may not preserve the following features.
OK, hold on tight, because we're about to get technical. Unlike the poetry of say, Shakespeare, which has a predetermined number of syllables per line (10), a line of Anglo-Saxon poetry doesn't have a set number of syllables. Instead, it has a set number of stressed syllables – four – with a slight pause in between the first and last two stresses, called a caesura. The first stressed syllable of the second half-line has to begin with the same sound as one or both of the stressed syllables in the first half-line. This repetition of beginning sounds is called alliteration. Take a look at the first lines of "The Wanderer":
Oft him anhaga [pause] are gebideð,
(Often the lone-dweller waits for favor)
metudes miltse, [pause] þeah þe he mod-cearig
(the mercy of the Measurer, though he unhappy)
These lines are a different number of syllables (10 and 12), but in both cases, there are four stresses in each, a pause in between the first and second half-line, and alliteration among the stressed syllables, with the long "a" sound of oft, anhaga, and are in the first line, and the "m" of metudes, miltse, and modcearig in the second.