Difference between functions using public _someName = () => {} and public _someName() {}

Question

I'm learning JavaScript/ReactJS and I'm finding I'm using functions with these variations:

public _someName = () => {} why the = before round brackets and why the lambda?

and

public _someName() {} What does this one NOT do compared to the above.

I've looked on SO and generally for some clear explanation of the difference but nothing has been found. Can someone tell me what the difference is based on my notes above.

Btw: I've read the docs!