This is a light or beeping sound that lets you know that you've reached the monitor's range limit. If you have a model without this feature, static might be the only indication that you're out of range. (A monitor's range can vary due to your home's size, its construction materials, and other factors.) The greater the range, the better--especially if you plan to take your monitor outside.
If you sleep in the same room as your baby or live in a small enough space that you can always hear or see what your baby is up to, you probably don’t need a monitor. Otherwise, most parents enjoy the convenience a baby monitor provides—instead of needing to stay close to the nursery or constantly checking on your child, you’re free to rest, catch up on Netflix or get things done around the house during naptime. Monitors can also double as a nanny cam to keep an eye on your child and their caretaker.