Opinions differ about whether infants experience the sensation of boredom. The website MedlinePlus lists boredom as a possible cause of crying in infants younger than 6 years of age, but according to a 2007 article from the National Association for the Education of Young Children, adults might be assuming infants are bored when they are not. However, it is clear that infants need stimulation to develop properly.
When parents or babysitters hear the baby crying, they usually first check to see whether she needs a change or is hungry. Once these possibilities are ruled out, they might give the baby a rattle or a toy to entertain her on the assumption that she is bored. Depending on how boredom is defined, she might indeed be bored, but her boredom might be caused more by not being able to move around than by needing a toy to play with.
If boredom is defined as uneasiness caused by a lack of adequate stimulation, then infants can certainly experience boredom. However, lack of stimulation is not the same sensation as the absence of entertainment. Infants don't necessarily need to be entertained constantly and it can even be bad for their brain development if they were. While it might seem as though a baby lying on her back and wiggling her arms is not doing much, she is working hard to learn how her body works and what she can do with it.
Caregivers sometimes keep babies contained in swings or baby jumpers to ensure their safety and distract them while the caregiver performs other tasks around the house. This can interrupt the baby's most important task, which is to learn how to move. When babies are born, they have so little control over their own bodies that they seem to twitch and shake involuntarily. During the ensuing weeks and months, they learn through trial and error how to move their arms and legs, open and close their hands and hold their heads up when they want to. This is essential for brain development and movement control.
When a baby is prevented from working on his ability to move, he can become frustrated and start crying. Take the baby out of her swing or whatever else is restraining her movement, put a toy or a few toys nearby in case she wants them and place her down on her back. Once she can move around and work on her movement skills, she might not need any other entertainment. If she still seems bored, you can play with her for a while.