We don't know what it is, so don't feel bad for having no clue! Even the author herself states in the poem, "I who don't know the secret wrote the line", and she loves the two girls for "finding what I can't find". So, two girls read a line in one of her poems, and were so moved by it that they felt that they had discovered "the secret of life in a sudden line of poetry." But Denise doesn't know what that secret is, or even what line it was in, because the girls don't tell her.
The main point of the poem isn't what the secret is, it is that people who read poetry interpret it differently; they read their own lives into the poems, and interpret them in ways that are meaningful to them. We filter the poem through our own perceptions, so a line that poet wrote (like Denise Levertov did) means something completely different than what the poet was thinking when they wrote the poem. That is okay with Denise, who states that she is happy that they can rediscover that secret "a thousand times...in other happenings" and that they felt such a profound connection to her poem in the first place.
I always thought the "secret" was the secret of creation, the source of life, and that the poem was saying that we perceive life and the source of life through the lens of our own experience. It's not that we invent the secret of life, but that we all perceive it with our own imagination and intelligence.