It appears to have many picture frames with SpongeBob's face in them, which correspond to the number of months SpongeBob has worked. Unlike Squidward, SpongeBob has worked hard for many months to have accumulated these awards.
Since these numbers exceed 12, 24, or even 36, it shows he has worked for many years.
In certain episodes, where this object appears, the number of picture frames can change, which decides how many years SpongeBob has worked at the Krusty Krab.
Also, in some episodes, the location of the object has changed. For example, in its debuting episode ("Employee of the Month"), in was on the wall of the Krusty Krab where Squidward's post is, but in "Breath of Fresh Squidward," it is shown inside the kitchen where SpongeBob works.