O no! it is an ever-fixed mark That looks on tempests and is never shaken;
Now that we’ve seen what love isn’t, we learn what the poet thinks love is.
In these two lines, he brings some nautical imagery into the mix (think storms and ships, not anchor tattoos and pirates).
In Line 5, he dramatically changes the tone with "O no!" to signal this shift from negative to positive, and immediately launches into an affirmation of love’s qualities. It is, as he says, an "ever-fixed mark" – that’s easy enough, it just means a marker that never moves.
Line 6 emphasizes this steady, solid quality, saying that it weathers storms ("tempests") but is never disturbed.
What kind of marker is it, though? The answer to this question comes in the second half of the quatrain.
It is the star to every wand’ring bark, Whose worth's unknown, although his height be taken.
Here, we discover that the "ever-fixed mark" that came up in line 5 is a star – not just any old star, but the North Star, the only one that never changes position in the night sky.
This refers to old-fashioned navigational knowledge; before the days of GPS and even reliable maps, sailors would chart their location in the ocean based on the position of the stars.
Line 8 also refers to these astronomical ideas. In the Elizabethan period, nobody knew what stars were made of (which is why the star’s "worth [is] unknown"), even though mariners did know the location of stars in the sky, or their "height."