Caesura, in verse, is a pause in a line that exists where a person would naturally pause while speaking. This pause can come at the beginning of a line (called initial); at the end of the line (called terminal); or in the middle of a line (called medial). Sometimes there is a punctuation mark present to indicate the pause, and sometimes there is not. I will transcribe the first several lines of the poem and insert a "||" symbol (which is a sort of poetic shorthand to indicate the presence of a caesura) where each of these pauses would occur.

So. || The Spear-Danes in days gone by ||
And the kings who ruled them had courage and greatness. ||
We have heard of those princes' heroic campaigns. ||

There was Shield Sheafson, || scourge of many tribes, ||
A wrecker of mead-benches, || rampaging among foes. ||
This terror of the hall-troops had come far. ||
A foundling to start with, || he would flourish later on ||
As his powers waxed || and his worth was proved. ||
In the end || each clan on the outlying coasts
Beyond the whale-road had to yield to him ||
And begin to pay tribute. || That was one good king. ||

You can see that a caesura accompanies commas and periods, which indicate where we should pause when we read aloud. Further, a caesura often precedes a conjunction like "and" or "but" or occurs after an introductory phrase like "In the end."