"Throw in the towel" is an idiom (saying). Derived from boxing or sports generally, it means to give up, or quit. / In boxing, throwing the towel into the ring is taken as quite literal, as they throw in the towel to stop the fight. / In more everyday use, you could use it on the sports field, or a student may use it describing a course or something. It's basically another way of giving up/quitting. It's quite often used when people retire "I'm going to throw in the towel now" or "I'm going to hang up my boots." / The / I put in, just indicates where my paragraphs end for you. Sometimes it doesn't show my paragraphs and just shows a big block of text.