Ophelia begins by handing out rosemary, which she says is "for remembrance." This remembrance could be for any of the dead or slain characters that have been produced by the play's plot thus far, or we as readers could infer that the remembrance refers to her once-love of Hamlet.
She proceeds then to pansies, which she says represent "thoughts." Thinking and pensiveness play heavy roles in this play, as demonstrated by Hamlet's melancholy state, among other things. These thoughts could be interpreted to be about any one of several possibilities at this point.
The fennel, columbines, and rue are supposedly "herbs of grace," used at funerals and other somber occasions. She also says that her receivers must wear their rue differently, as she is mourning the death of her father.
Lastly, Ophelia gives a daisy, ordinarily a happy flower, but she states that she "wishes (she) had violets, but they withered when my father died." The symbolism here should be obvious.
Every flower provided has its own meaning, but in each case, the reader can detect double entendre and a sickening sense of metaphor for all of them. A little internet detective work on the meaning of flowers will also help the reader.