Miriam and Aaron began to say bad things about Moses. That's because Moses had married a woman from Cush.
References for Numbers 12:1
"Has the LORD spoken only through Moses?" they asked. "Hasn't he also spoken through us?" The LORD heard what they said.
Moses wasn't very proud at all. In fact, he had less pride than anyone else on the face of the earth.
The LORD spoke to Moses, Aaron and Miriam. He said, "All three of you, come out to the Tent of Meeting." So they did.
Then the LORD came down in a pillar of cloud. He stood at the entrance to the tent. And he told Aaron and Miriam to come to him. Both of them stepped forward.
Then the LORD said, "Listen to my words. "Suppose one of my prophets is among you. I make myself known to him in visions. I speak to him in dreams.
But that is not true of my servant Moses. He is faithful in everything he does in my house.
With Moses I speak face to face. I speak with him clearly. I do not speak in riddles. I let him see something of what I look like. So why were you not afraid to speak against my servant Moses?"