Basically, it means the same as day time potty training, i.e. when your child has the urge to urinate; they would go to the potty to urinate.
However, achieving night time control is not simply a learned skill, but rather a physiological development and the control is largely involuntary. In some cases, circumstances require learning how to achieve night time control

Babies urinate around the clock and then at about the average age of 18 months, as the sphincter muscles mature, toddlers will make the transition from urinating around the clock to only urinating during the waking hours as is the case with most adults.

Having to urinate at night is not an issue with most adults, since when an adult's bladder is full at night, a signal goes from the bladder to the brain and we are awakened with the need to go. For children, this signaling mechanism comes with age.

Not the Same as Bedwetting

When parents decide it’s time to begin toilet training, they usually look at the developmental process as a whole. pediatrician at the Medical College of Georgia Children’s Medical Center, says by age 2, most children are aware that the bladder is full, but it isn’t until they are 4 years old that they learn how to pee on demand.

“During the day, the child is awake and able to respond to the full bladder,” Dr. says. A sleeping child will not notice the full bladder, and because most children are potty trained before their bladders are fully developed, they aren’t always able to completely empty their bladder before bed. In other words, if they don’t have to go before bedtime, they may not be able to go.

It’s important for parents to remember that nighttime wetting during the potty training process is not the same as bedwetting.

“We do not classify children as bedwetting until they are 5 years old,” says Dr. Bauza. “Parents need to remember that it can take up to two years before a child is completely trained at night.”

Pediatric and urology nurse practitioner at the University of Colorado Denver Health Services Center, says the bladder functions best when it is emptied between eight and 10 times a day. However, most children don’t use the potty that often. A child who is working on potty training might run to the bathroom two dozen times a day, but rarely do they actually pee, and even then, they don’t always completely empty their bladder.

“If the child is peeing frequently and fully during the day, the nighttime stuff will come more naturally on its own,” Passamaneck says.

• Be aware of the child’s readiness. At night, children should feel comfortable getting out of bed and finding the bathroom alone. If the child can’t get out of bed without help – for example, if they have a rail to prevent them from falling out of bed – they probably aren’t ready to begin nighttime training.
• Transition from diapers to PULL-UPS® Training Pants is made specifically to help children with this transition). “A child will feel more confident if he can use PULL-UPS® Training Pants at night,” says Dr. Bauza. When the Training Pants are regularly dry at night, that’s the time to transition to underwear.
• Use a mattress cover.
• Try to limit fluids before bedtime.
• Make a trip to the potty right before turning out the lights part of the bedtime routine. Also, encourage the child to use the potty as soon as she wakes up. This will help put the bladder on a schedule.
• Be patient and reassuring. “It’s not a big deal to achieve dryness at night until the child is older,” says Dr. Bauza.

While some experts suggest waking the child at night for a bathroom trip, others think it defeats the purpose, as the idea is to help the child learn to feel natural bladder urges. If the parent is unsure, ask the child if she wants to be woken. As with other aspects of potty training, it is good to let the child take the lead.