The visualization in the last line refers to us (the wakes or waves) being part of a whole (the sea). Advaita teaches us the same, that we (including inanimate objects) are all one. Advaita means non-dual, that you and the higher power is one and the same thing. Just like in language, where an experience was broken down into pieces and assigned labels, many concepts are artefacts of dividing our experience into pieces and assigning labels: 'me vs others', 'free will vs determinism', 'now vs past and future'. When oneness is perceived, many of these concepts fuse together and we stop dividing our experience into its parts.

An excerpt from Tuesdays with Morrie (192 pages, 2002) by Mitch Albom, explains the relationship between waves and the ocean:

A little wave is bobbing along in the ocean, having a grand old time. He is enjoying the wind and the fresh air - until he notices the other waves in front of him, crashing against the shore.

"My God, this is terrible," the wave says. "Look what's going to happen to me!"

Then comes another wave. It sees the first wave, and it says to him, "Why do you look so sad?"

The first wave says, "you don't understand! We are all going to crash! All of us waves are going to be nothing! Isn't it terrible?"

The second wave says, "No, you don't understand. You are not a wave, you are part of the ocean."