As others have written, the ס (Hebrew letter samekh) stands for the Hebrew word “setumah”, that is, “closed” or “blocked”. This indicates that in the writing of the Torah scroll a space is to be left blank to mark the end of a section, but the line is to be closed, that is, the next section is to begin on the same line (thus the line is not left “open-ended”). Where there is one ס, the space to be left open before beginning the next verse must be equivalent to the space of four letters. If there are three סs, then at least 12 letter spaces are to be left unwritten before beginning the next verse on the same line.
Similarly, the פ (Hebrew letter peh) stands for the Hebrew word “petuchah”, that is, “open”. That indicates that the end of the section must be indicated by leaving the entire rest of the line blank.
