The speaker is talking to someone who has stopped feeling like a real person. Instead of a living, breathing human, this person has become "words in the sand": something written down, fragile, and easy to wipe away. The poem doesn't say why. It could be a lost love, a friend who drifted off, or someone the speaker has deliberately pushed out of their heart. The poem leaves that open.
The image of the tide does most of the work. When it comes in, the words vanish without a fight, and that is how the speaker describes forgetting this person. There's a coldness in it, since turning someone into mere writing is a way of keeping them at a distance. Yet the short, halting lines, broken up by commas, also hint at sadness or a struggle to let go. The mood is quiet, detached, and a little wistful, like watching something you once cared about slowly wash away.