At first, this looks like a simple lesson about flowers. Dandelions and daffodils are both yellow and pretty, but the speaker says one is "true" and the other isn't. Dandelions are weeds, pulled out by the root, while daffodils, with their bell-shaped centers and link to new beginnings, are called "beautiful through truth." The catch is that you can't tell the difference by looking. You only know because someone told you.
That's where the poem gets interesting. The flowers seem to stand for something bigger, perhaps the way things or people can look the same on the surface while being judged very differently. The speaker seems to sense how arbitrary that is, and the last two lines, "I dont know / I think too much," show a mind second-guessing itself. The mood is quiet, thoughtful and a little uneasy, like someone caught between accepting a rule and questioning it.