What is the meaning of ヌ?

The katakana syllable ヌ (nu). Its equivalent in hiragana is ぬ (nu). It is the twenty-third syllable in the gojūon order; its position is ナ行ウ段 (na-gyō u-dan, “row na, section u”).

Iteration mark, indicating that the previous character should be repeated. The Japanese equivalent is 々.

Source: wiktionary.org