This is the 1406th most frequent Catalan word.
lletra
"Letter" (of the alphabet).
The term 'lletra' here is used to refer to the words of a song, often translated as 'lyrics'.
La lletra de la cançó és molt emotiva.
The lyrics of the song are very emotional.
In this case, 'lletra' refers to a character of the alphabet, such as 'a', 'b', etc., emphasizing meticulous writing.
N'has tingut cura de posar cada lletra amb perfecció.
You took care to write each letter with perfection.
Here, 'lletra' is used to mean the font size or type of letters in printed material.