In "The Secret," by Denise Levertov, the narrator tells about two girls who "discover the secret of life in a sudden line of poetry." Although the narrator (presumably Levertov herself) is the author of the lines that the girls discover, she does not know the secret. The girls themselves have already forgotten "the secret, the line, the name of the poem."
The narrator still loves the girls and does not care that the girls have forgotten "the secret of life." What is more important is their "wanting to know," and their "assuming there is such a secret."
The poet is saying that the search for meaning in life is more important than the actual finding of the meaning. The meaning itself can change from time to time as a person grows and changes situations. What matters is that the person is thoughtful and questioning.
This is a poem about discovering the power of poetry. We could describe this power as imaginative power or as the connective, inspirational/inspired or intellectual/stimulative power of poems. The girls featured in the poem represent an awareness and appreciation of poetry's potential to function as surprise, as valuable experience and thus as "life," in a certain way of speaking.
"For [Levertov], to write poetry is not simply to manipulate words" (eNotes).
The girls' discovery recounted in the first stanza is not necessarily a discovery of meaning. It is not a specific discovery about what a line of a poem was trying to say. Rather, the discovery is related to the idea that poetry has life. It has power.
Two girls discoverthe secret of lifein a sudden line ofpoetry.
[...] they maydiscover it again, in otherlinesin otherhappenings.