In an effort to embarass Malvolio, Maria writes a letter in Olivia's handwriting, telling him that she is in love with him. The letter is only signed with what seems to be the letters of an initial. Here, Malvolio reads lines of the letter aloud, which he will later deconstruct, in order to find out if it was Olivia that wrote it. This line is the first major hint that it could be Olivia. The person who wrote the letter both commands and loves the same person, suggesting that it is a master who loves her servant (Malvolio).
The love letter is a symbol of the performative nature of love seen throughout Twelfth Night. Characters feel that the only way to properly share their love or express it is through extreme and over-the-top behavior, whether that be song, poems, laments, or letters. So, while this letter comes at a surprise for Malvolio, he is not put off or shocked that someone would express their love in this way. This is also another moment of deception in the play, as Maria uses deception as a mechanism to embarrass and punish Malvolio for his mistreatment of other servants and his self-righteousness.