What is this white stuff that comes out of chicken when I bake it?

The white stuff that comes out of chicken when baking or frying is completely normal and consists mainly of protein, which coagulates when heated.

To be more specific:
Meat contains a lot of water and protein (especially myoglobin and other muscle proteins).

When heated, the muscle cells contract and squeeze out water and dissolved proteins.

These proteins (especially albumin) coagulate when exposed to heat, forming the white, sometimes foamy or solid material you see.

It’s especially common when the chicken is cooked very quickly or at high temperatures.