"一" has no meaning, it often appears in katakana (there are two kinds of kana in Japanese: hiragana and katakana).

For example: センター (center) is a foreign word, transliterated from the English "center", meaning "center".

The "一" here simply means the long sound of "タ". The "タ" here has two syllables, which is a long sound.

Note: The homophony of "タ" is similar to "he" in Chinese.

I hope you can accept this explanation.