Whole note

Four quarter-note beats: in common 4/4 time, one whole bar. It is the longest note in everyday notation. Two half notes (𝅗𝅥) or four quarter notes (𝅘𝅥) fill the same time. Its pitch comes from where the oval sits on the five-line staff; the stand-alone character has no staff, so it shows only the length.

The value goes back to medieval measured notation of the late 13th century, where the semibreve was a division of the breve (at first usually a third of it) and was written as a small diamond; the round hollow head came later. Back then these were quick notes, so the "short" breve earned its name; as ever-shorter values were added below them and the beat moved to smaller notes, they became today's long ones.

Why do I see a box instead of the note?

Your font lacks this rare music character. Many ordinary fonts don't cover the Unicode musical-symbols block. The meaning is unchanged; install or pick a font that covers music symbols, or view it on a device whose font has it.

For real sheet music, use notation software rather than text characters: it places notes on the staff with pitch. 𝅝 suits labels, worksheets and posts about rhythm.