Don't really understand this concept that my teacher explained to me today and these notes are all i have of it

Given:
[tex]f(x)=x^2[/tex]When we substituting the values of 'x'
[tex]\begin{gathered} f(0)=0^2=0 \\ f(1)=1^2=1 \\ f(2)=2^2=4 \end{gathered}[/tex]