What is the meaning of $A+B$ when $A$ and $B$ are two finite fields?

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I have read such a statement: let $\mathbb K$ be a function field of one variable over the finite field $\mathbb F$. Let $f(x)=x^p-x$, $p$ being the characteristic of $\mathbb K$. Then there is some statement about $f\mathbb K+\mathbb F$, but I don't know what the author means by this. I guess it is a stupid question, but I am a physics student and not very familiar with abstract algebra. Can anyone help?