F is a function. It takes inputs and then produces an output.
The notation <span>f(x)</span> is saying more specifically that the function is taking an input x and producing the output <span>f(x)</span>.
For example, if you look at <span>f(x)=<span>x2</span></span>, you can think that f is the "squaring function", with input x and outputs <span>x2</span>.
You can think that <span>"f"</span> is like the name of the function.
<span><span>f1</span>(x)=<span>x2</span></span>
<span><span>f2</span>(x)=<span>6x</span>+3</span>
Above, you know which function I'm talking about if I say <span>f2</span>. But often, people will just call it as the function <span><span><span>f2</span>(x)</span></span>
