Gershayim
Gershayim (U+05F4 in Unicode, and resembling a double quote mark) (sometimes erroneously referred to as merkha'ot, which is Hebrew for double quote) are inserted before (to the right of) the last (leftmost) letter to indicate that the sequence of letters represents a number rather than a word. This is used in the case where a number is represented by two or more Hebrew numerals (e.g., 28 → כ״ח).
Similarly, a single Geresh (U+05F3 in Unicode, and resembling a single quote mark) is appended after (to the left of) a single letter to indicate that the letter represents a number rather than a (one-letter) word. This is used in the case where a number is represented by a single Hebrew numeral (e.g., 100 → ק׳).
Note that Geresh and Gershayim merely indicate "not a (normal) word." Context usually determines whether they indicate a number or something else (such as "acronym").
Read more about this topic: Hebrew Numerals