The ELI5:

1. Big rock from sky hit Earth.
2. Earth get hot, then super dusty for long, long time. Dusty Earth block light from sun.
3. Plants die. Veggy Dinos no longer have lunch. They die.
4. Meaty Dinos no longer able to lunch on Veggy Dinos. They die.
5. Only tiny flying Dinos survive. Tiny flying Dinos become Birds.

The end.