They are treats made for amoebas and come in a brown paper bag that depicts the word "amoeba" in red text and the word "treats" in bluish-purple text bordered by a red outline. Below the text is an illustration of the food inside. Speaking of which, the food is shown to be white bone-shaped treats.
Role in episode
After Plankton examines his pet amoeba, Spot, with an x-ray, he is shocked to find out that he's pregnant, assuming he was a boy. Karen points out that it doesn't since the provision of food-rich environments causes amoebas to reproduce. The scene then pans to all the food Plankton has been feeding to Spot lately and one of them is Amoeba Treats.
Towards the end of the episode, after Plankton's attempt to steal the Krabby Patty secret formula by using Spot's children to gain possession of it fails (as usual), SpongeBob pours a bag of Amoeba Treats on Plankton as revenge for trapping him in a cage, which attracts several of the children and increases their size.