This is a musical note called a quarter note (or crotchet). It lasts for one beat or count.
The quarter note is made up of two parts, the head and the stem. The stem can go up or down. If the stem goes up, it is joined to the right side of the head. If the stem goes down it is joined to the left side of the head. Notes written below the middle line of a staff usually have their stems going up.
Notes written below the middle line of a staff usually have their stems going up. The stem for the note B on the treble staff can go up or down. The stem for the D note on the bass staff can also go up or down.
The note B is written on the middle line of the treble staff.
The note D is written on the middle line of the bass staff.
Using a notebook or some paper, practice drawing quarter notes on the line. Draw one with its stem going up and the next one with its stem going down. Draw ten quarter notes altogether - five with the stems going up and five with the stems going down.