You have probably heard the phrase "sleep begets sleep," perhaps from an older relative or an experienced mum at playgroup. It's a confusing one, partly because it's so counter intuitive. After all, surely a baby will sleep better if they are tired!
In a way you are right, nobody can sleep all the time and there is certainly a point at which you can't sleep at night if you slept all day! Having said that, most babies and toddlers are running short on sleep so if you are having difficulties at night it is highly unlikely that cutting down on naps will be the answer.
Our bodies are complex things and there are many influences on them, but there are two main reasons why being well rested makes it easier to fall asleep and stay asleep. Both are to do with hormones. This is a very brief summary since it's not intended for scientists, and I'm not one either! But on a practical level it can help to have a basic understanding.
First, levels of the "sleep hormone" melatonin go up when you are asleep. In an adult they go up in the evening and fall again in the morning, staying very low all day. When a child has lots of naps the level hasn't so much time to drop at each waking and stays a bit higher. Melatonin is so effective in helping your child to fall asleep that it is sometimes prescribed to people with sleep problems, so might as well make use of it when it occurs naturally! If your little one hasn't had a nap in a while their melatonin level will be lower, making falling asleep harder.
To add to that, levels of the "stress hormone" cortisol drop during sleep. So a well rested child is likely to feel calmer and more relaxed, exactly the state you need to fall back to sleep at bedtime. It is important to know that cortisol is entirely normal, and even essential. The levels we are talking about here are part of normal daily fluctuations and not high enough to cause brain damage or other problems (those are usually reserved for really traumatic situations, like war). Even so, a slightly high cortisol level can make falling asleep tricky!
So, there it is in a nutshell. Sleep begets sleep because the hormone changes during sleep make it easier to fall asleep, and stay that way. Which is why I sometimes suggest an earlier bedtime, or longer nap, for a little one who struggles at night.