"T.L." is used to refer to Mrs. Tilford. Mary is trying to intimidate Rosalie to back up the allegations about Martha and Karen.
This cryptic abbreviation derives from an exchange between a father and his sons in 1605.
One day, Edmund of Langley saw that his sons were studying their Latin. According to an account of the incident, Langley asked his sons if they knew the Latin word for “fetter-lock.” When it became evident that they didn’t, the father said , “I will tell you!” adding “Hic hac hoc taceatis” by way of advising them not to tell their instructor that he had coached them. [The word “Tace” is Latin for “Be silent.”]
Either Langley or one of his sons (probably the latter) later recounted this story. In academic circles, it soon became a common example of helping someone (or doing someone a favor) secretively.
In his novel AMELIA, Henry Fielding refers to this account when one character cautions another, “Tace, Madam, is Latin for candle. I commend your prudence” - identifying the word “tace” with something done or said “in the dark.”
In 1811, Sir Walter Scott hearing the phrase for the first time, said: “Tace will be hereafter with me Latin for ‘candle!’” Students and academics soon adopted this phrase [“Tace is Latin for a candle”] - which was eventually abbreviated to TL - as a veiled hint to something about which one must remain silent, i.e. a secret.
T.L. means telling the other a secret. I'm not exactly sure what it stands for but it's something along the lines of sharing a secret. ie, Rosalie told Mary a secret about how Lois thought she was smart, I think? I did that scene in Drama and T.L. was used, although I'm not sure whether this is the scene you're referring too.