Top of flight refers to the highest point of a plane's journey. When you go on a plane ride, the plane takes off from the ground and goes up, up, and away into the sky. As the plane goes higher and higher, it eventually reaches its top of flight, which is the highest point the plane will go in the air before it starts to come back down.
You can think of it like going on a swing set. When you start swinging, you go back and forth in smaller and smaller arcs until you reach the top of your swing. That's the highest point you will reach before you start to swing back down again. The same thing happens with a plane – it goes up and up until it reaches the very top, and then it starts to come back down to land.
The top of flight is also important for pilots to know because it helps them plan their flight path and make sure they have enough fuel to get to their destination. They need to know how high they are going to go so they can calculate how long it will take to get there and make sure they have enough fuel to make it.
So, in summary, top of flight is the highest point a plane reaches on its journey before it begins to descend back down. Just like the top of a swing set, it's the highest point the plane will go before it starts to come back down to earth.