幸福 means “happiness” or “bliss,” with a slightly more poetic and profound feeling than the everyday word 幸せ. In the song, it appears in the image of “a small happiness blooming in the heart,” turning an ordinary moment of comfort into something precious and memorable.
This is a goodnight song, sung straight to someone at the end of a long day. It opens with "今日は疲れたね" (You're tired today, huh) and then notices something nice: "でもどこかうれしそうだね" (but you somehow look happy). The singer wants the story, "きかせてよ" (tell me about it), and guesses the reason: you were a little kinder than usual today and feel shy about it, "優しくなれた自分に テレながら" (blushing at the self who managed to be kind). テレる means to feel embarrassed in a sweet, bashful way, and it is a lovely word to learn. That good mood spreads until even "道ばたに咲いた小さな花まで" (even the little flowers blooming by the roadside) look beautiful.
The second verse flips it. Now the day went badly: "イヤな事でもあったの? 話してよ" (Did something bad happen? Talk to me). The person is crying over a self they dislike, and the singer does not try to fix it. Instead you get the gentlest line in the song, "誰も強くなんかない" (nobody is really strong), followed by the promise that tomorrow you can be "素敵な自分" (a wonderful you). The chorus is a bedtime ritual: let the night melt the day away, "夢で逢おうね" (let's meet in our dreams), and show me a good smile tomorrow too. My favourite bit is the second chorus, where the singer says "頬濡らしたままでもいいから" (it's okay even if your cheeks are still wet) before "もう、おやすみ" (now, good night). It is soft to the point of being a bit sugary, but it is also really useful Japanese: casual endings like ね and よ, and phrases you would actually say to someone you care about.