Pluto is a dwarf planet in our solar system. A dwarf planet is not a planet, but it looks a lot like a planet. The planet Neptune had been studied for a long time: its orbit around the sun seemed a little strange. It was assumed that there must be another celestial body that disturbed Neptune's orbit with its gravity. In fact, it was found in 1930 where it was expected. At that time, however, Pluto was still counted as a planet.
The name was chosen by an eleven-year-old girl: Pluto is the god of the underworld, the dark realm of the dead. That seemed fitting for a celestial body that is so far away from the sun.
There is much that is special about Pluto. The orbits of the eight planets all lie in the same direction, but Pluto's is at an angle to it. Also, more than the planets, its orbit is shaped like an egg: sometimes Pluto is even closer to the Sun than Neptune and sometimes much further away. Pluto takes over 247 years to orbit the sun once. That would be a "Pluto year".
As far as is known, Pluto has five moons. One of them, Charon, is about half the size of Pluto itself. That is also very unusual. Some researchers therefore think that Charon is not a moon of Pluto at all. They say Pluto and Charon are actually two celestial bodies orbiting each other and would prefer to call them "double dwarf planets", but Charon is still considered a moon of Pluto.
In the years before 2006, astronomers discovered more and more small celestial bodies that they did not want to call all "planets". That's why they thought twice about it. Since then, Pluto has only been considered a dwarf planet, just like Ceres, for example. This is not only because it is much smaller than previously thought. There are other celestial bodies in its orbit that a real planet would have swept away long ago.