Don't feel bad. I didn't really understand them either when we first moved here. Tie backs are an anchoring system for posts along the bulkhead. Even though the posts along the bulkhead and the 2 posts we installed for the lift are really deep in the ground, the ground is soft and there is constant current that causes movement and erosion. Also, the weight of the earth against the bulkhead causes tons of pressure on the posts and for the boat lift posts, the weight of the boat puts stress on the them.
To help with strength, the poles are "tied back" into the yard. Posts along the bulkhead are connected to a buried post in the yard with a metal rod.
Here is how we anchored our boat lift posts.
First we measured the distance of the metal rod from the boat lift post into the yard. Then dug holes in the yard that lined up with the posts. Thank you to our friend for helping with this not so fun step.
Then we drilled a hole through the boat lift post and through the anchor post.
We put the 4 ft posts in the ground and then poured in concrete. When the concrete cured, we tightened the bolts and buried the post.
The walkway for the dock will cover the rod and french drain. You will never know there are tie backs except for the bolts in the boat lift posts.
Put A Bird On It, DIY Showoff, Remodelaholic, Life in Velvet