The line you're referring to is the third line in Byron's poem "The Destruction of Sennacherib." It appears in the first stanza, which I've pasted below:
The Assyrian came down like the wolf on the fold,
And his cohorts were gleaming in purple and gold;
And the sheen of their spears was like stars on the sea,
When the blue wave rolls nightly on deep Galilee. (1-4)
The line "the sheen of their spears was like stars on the sea" is meant to convey the vast size, power, and might of the Assyrian army. The line achieves this effect in a couple of ways. First, by comparing the shining spears to stars, Byron illustrates just how many spears (and, by extension, how many soldiers holding said spears) there are in the Assyrian army. Like the stars in the sky, the Assyrian army appears endless. Second, Byron also uses this line to describe how majestic the Assyrian army is. Just as the stars are full of celestial beauty, so too is Byron's Assyrian army dazzling and totally awe-inspiring. All in all, the line you're asking about here is meant to convey the impressive extent of the Assyrian military might.