Step climb is like climbing stairs but with an airplane. Just like when you walk up stairs, the airplane goes up in steps instead of all at once.
When the airplane takes off, it is usually at a lower altitude because it needs more power to lift off the ground. As the airplane flies further away from the ground, it becomes easier to fly because there is less air resistance.
Now imagine that the airplane wants to fly higher in the sky to reach its destination faster or to avoid bad weather. Instead of flying straight up, which would use a lot of fuel, the airplane will climb up in steps.
First, the pilot will increase the power and angle of the airplane so it climbs a little bit higher than it was before. Once it reaches that first step, the pilot will level it off and the airplane will fly straight for a while.
Then, the pilot will do it again, increasing power and angle to climb up to the next step. This process will continue until the airplane has reached its desired altitude.
Just like climbing stairs, step climbing is more efficient because the airplane uses less fuel than if it tried to climb straight up. Plus, it's safer because it gives the airplane time to adjust to the changing air pressure.