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 alliterate with (start with the same sound as) one or both of the stressed syllables in the first half-line. Take a look at lines 24-25 of "The Husband's Message":
Ne laet þu þec siþþan [caesura] iþes getwaefan,
lade gelettan [caesura] lifgendne monn.
Afterward, do not let any living man
keep you from the voyage or hinder your journey.
These lines are a different number of syllables (11 and 9), but in each case there are four stresses, a pause between the first and second half-line, and alliteration on some of the stressed syllables, with the 's' of siþþan and siþes linking together the half-lines of 24, while the "l' of lade, gelettan and lifgendne does the job in line 25.