Two people, one day, and nothing more is asked of it. That is the whole of this tiny poem, and its calm comes from how little it wants. The speaker says that today "you" exist and today "I" exist, then stops to say that this is enough. There are no promises about the future, no regrets about the past, and no list of what might be missing.
The word "today" is repeated three times, and it keeps the speaker's attention fixed on the present. The grammar is slightly off ("you exists"), which gives the line a soft, unpolished, almost whispered feel, as if it were a private thought rather than a performance. We aren't told who "you" is, so it could be a lover, a friend, or even the reader. The mood is quiet, grateful and gentle. It suggests relief at simply being alive alongside someone else, and the short lines and wavy tildes make that feeling seem small, light and easy to hold.