Imagine you are making pancakes with a pan on the stove. When you pour the pancake batter onto the pan, it starts to spread out and flatten. Sometimes when you cook the pancake, it rises up a little and becomes a bit fluffy.
Now imagine the Earth is making pancakes, but instead of batter, it is using lava. When a lot of lava comes out of a volcano, it can spread out and create a big flat area called a lava flow. But sometimes, the lava is so thick that it can't spread out easily. So it starts to pile up on top of itself, kind of like how some pancake batter can stack up in the pan.
When the lava piles up high enough, it can create a dome shape, just like how some pancakes can form a dome in the pan when they rise. And when that dome cools and hardens, we call it a pancake dome. So basically, a pancake dome is a big pile of lava that looks like a dome-shaped pancake!