My understanding is that the problem has to do with the definition of a planet. A planet must:
1. Orbit a star
2. Have a strong enough gravitational force that it forms a round shape
3. Be the “dominant” mass in the area of its orbit. That is, if it passes by any other objects by a certain distance while orbiting the sun, then those objects should get trapped in the planet’s gravity
Pluto doesn’t meet the third criteria. There is basically “stuff” around Pluto that Pluto isn’t big enough to pull into its orbit. So Pluto fails to meet all criteria for a planet.