In season three of "Parks and Recreation," Donna Meagle reveals that she has been selling rubber nipples since the government shut down.
The joke appears in "Go Big or Go Home," the season 3 premiere. Pawnee's government has been closed during a budget crisis, leaving the Parks Department employees temporarily without their usual work. When Leslie Knope checks in on her colleagues, each person has found an unexpected way to spend the shutdown. Donna's temporary job is answering phones and taking bulk orders for rubber nipples.
The line fits Donna's characterization: she is private about her personal life, financially comfortable, and clearly capable of pursuing unusual side work without explaining herself. The comedy comes from the contrast between her normally glamorous presentation and the bizarre product she is selling.
The shutdown is fictional and concerns Pawnee's municipal government, not a real U.S. federal shutdown. Another source of confusion is that rubber nipples are also connected to the fictional Sweetums company's origin story, but Donna's specific shutdown job is selling them by telephone.