Queen Margaery Tyrell has narrowly escaped a walk of atonement, but in exchange, she must covert her grandmother, the Queen of Thorns. Margaery manages to slip her an image of a rose just before urging her to go back home, but what does Margaery's note to her grandmother mean?
For the past few episodes — basically since she saw her brother Loras beaten up in a dungeon, begging her to end his suffering by whatever means necessary — Margaery seems to have drunk The High Sparrow's Kool-Aid. She even convinced Tommen to join The Faith and The Crown in an alliance that very much dissolves any semblance of separation of church and state that Westeros previously enjoyed. The High Sparrow is, of course, nothing more than a religious zealot, and now Margaery has essentially doomed the entire kingdom to his fanaticism.
Her grandmother is particularly enraged, since she had just assembled all the military strength of House Tyrell, along with Jaime Lannister and his men, to put a stop to The High Sparrow and overthrow him once and for all. But with The Crown now joined with The Faith, anyone who speaks out against the latter is committing treason.
While Margaery has put on a persuasive enough act to convince her husband, King Tommen, to formally entwine The Crown and The Faith, we know how cunningly she can be to get what she wants. In this case, Loras' release seems to be the most important thing to her. His impassioned plea for mercy had a huge impact on her, and, in an effort to comfort him, she promised she would stop fighting The High Sparrow to get them both out of the dungeons.
In the scene in which she is reunited with her grandmother, Margaery insists that the Queen of Thorns must repent (thanks to a thinly veiled threat from The High Sparrow), and craftily works in an urge for her to leave High Garden and go home, presumably for protection. But there is a suspicious septa who won't leave Margaery's side for the conversation. Margaery is clearly being watched, so she surreptitiously presses a note into her grandmother's hand. When Lady Olenna is alone, she opens it and finds a drawing of a rose, the message behind which she seems to understand.
Loras Tyrell is commonly known as The Knight of The Flowers, so the note might be a reference to him, to the idea that he must be kept safe, or a hint that Margaery is behaving the way she is to rescue him. Whatever Margaery means by it, her grandmother seems to understand that she isn't the religious zombie she's portraying herself to be, which is a relief to everyone in The Realm, along with viewers everywhere.