What is the meaning of ?

The hiragana syllable (se). Its equivalent in katakana is (se). It is the fourteenth syllable in the gojūon order; its position is (sa-gyō e-dan, row sa, section e).

one's back (part of the human body)

one's height

the rear of something, the area behind

rapids, current

thus

se (Japanese unit of area)

Source: wiktionary.org