The meaning of a poem is the experience the speaker (of the poem) or the author communicates. While, the idea is the underlying concept of the poem. Something which can be a generalization of live value. But, idea is not a message, since poems communicates experience more rather than gives moral message. I think the only way to understand meaning and idea is by asking two questions. For meaning,
what experience is the speaker telling about? --> e.g. A man walking in the wood and has to choose one of two roads (The Road not Taken)
What concept of life is implied in the poem? --> e.g. Man should always choose between two (any) choices in his life
what experience is the speaker telling about? --> e.g. A man walking in the wood and has to choose one of two roads (The Road not Taken)
What concept of life is implied in the poem? --> e.g. Man should always choose between two (any) choices in his life