Please explain the content, context, and tone in the poem “They Told Me You Had Been To Me.”

The poem, "They Told Me You Had Been To Her" is from chapter 12 of Alice's Adventures in Wonderland. The White Rabbit produces it as evidence against the Knave, who stands accused of stealing the Queen's tarts, claiming that it is "a set of verses" written by the Knave. When the King discovers that the verses are not written in the Knave's handwriting, he proclaims that the Knave "must have imitated someone else's hand."
Both the content and the tone of the poem are deliberately nonsensical, in large part owing to the excessive use of pronouns. The first verse, for example, consists of only four lines but includes the following seven pronouns: "They . . . you . . . her . . . me . . . him . . . She . . . I." This is very much a deliberate effort on Carroll's part to prevent the reader from following or comprehending the intricate web of relationships between all of the people alluded to by those pronouns. The first stanza, in fact, immediately disabuses the reader of any notion that there might be any decipherable meaning in this poem:

They told me you had been to her,And mentioned me to him:She gave me a good character,But said I could not swim.

The nonsensical tone here is created not only be the baffling accumulation of pronouns (with four different pronouns in the opening line alone), but also by the rhythm of the lines, which is similar to the repetitive, lilting rhythm of a nursery rhyme. This rhythm is created by the alternating end-rhymes ("her . . . him . . . character . . . swim") and also by the regular syllabic meter, with the first and third lines having eight syllables each and the second and fourth lines having six syllables each. This nursery rhyme rhythm helps to compound the sense that this is a childish, playful, meaningless "set of verses."
In the context of the narrative, this nonsensical poem is important because it helps Alice to realize that in this world, nothing makes any sense and all is "stuff and nonsense." This realization is what prompts Alice to emerge from the dream, thus bringing the story to a close. As soon as she realizes that she is in an illogical, meaningless dream, the spell of the dream is broken, and she wakes up "lying on the bank, with her head in the lap of her sister."