It totally depends on the word. It may be "singular vs. plural", as Majestic-san explained, or might change the meaning completely. That's just a mark. There are words that the same kanji are repeated in it, and the mark is used to represent the second kanji.
There also are other marks for kana or multiple characters. The ones other than 々 are not commonly used nowadays, as the wiki page linked below mentioned (only the (horizontal) kanji iteration mark (々) is commonly used today), though.