What is the meaning of 五十音?

The modern ordering of the Japanese syllabaries.

all Japanese morae that can be spelled with 45 single modern kana and 2 historical kana (/ and /) customarily without diacritical marks ( and ), excluding and , arranged in a 5x10 table (五十音図 (gojūonzu, literally table of the fifty sounds)), used for phonological and grammatical analysis

Source: wiktionary.org