Counter For Beats
counter
The kanji refers to a beat and the 〜 lets you know this is a counter. So this is used as a counter for beats for when you're listening to or playing music.
はく
This has the same reading as the kanji you learned, meaning you know the reading!
4分の4拍子の曲では、通常、1拍目と3拍目の頭に強拍が来ます。
For music in four-four time, the stress usually comes to a head at the first and third beats.