the ー makes the preceding vowel long, so ボール is pronounced as "booru"

You are welcome!

Thank you so much ^_^

Hi. Albert's answer is great. "ー" is generally used for the words from overseas written in katakana. Other examples are: コーヒー(koohii = coffee)、ケーキ(keeki = cake)、フルーツ(furuutsu = fruits)、メール(meeru = mail)、ノート(nooto = note)