A lacustrine plain is like a big, flat area where a lake used to be. Imagine you have a big bowl of water in your backyard and after a while, you take the water out, but the bowl is still there. The ground where the water used to be is a lacustrine plain.
Sometimes, a long time ago, there used to be a big lake where there is now a flat stretch of land. Over time, the lake water would fill up the bowl-like area and stay there long enough to create a deep lake. But one day, things changed and all the water went away. Maybe the water flowed away to another place or maybe the sun was really hot and the water evaporated into the air. Whatever the reason, the end result is that the large, deep lake dried up and left behind a flat plain.
These lacustrine plains can look like a big, empty field or pasture, and sometimes they have really fertile soil that is great for farming. Other times, they are barren and empty of any vegetation. Either way, a lacustrine plain is an area that used to be covered by water but now it's a big, flat, dry place to play, work, or explore.