The sentence describes two sailing ships, one following the other. A brig is a sailing ship with square sales. She is sailing behind the other ship (in her wake). A sloop is a saling ship with fore and aft sails, like a yacht. The brig is having difficulties keeping up with the sloop (straining to hold her position). The brig is low in the water, meaning either she is leaning over in the wind or she is carrying too much cargo.

Great answer from Drew. One thing to add: here, “lines” = ropes, such as ropes that hold sails in place. The lines are creaking (probably) because the wind is putting a lot of strain on them.