Suppose \(f(x)\) is a function of \(x\) and \(g\) is also a function of \(x\), then \(f(g(x))\) and \(g(f(x))\) are known composite function.
For example if \(f(x)\) is \({x^2}\) and \(g(x)\) is \(\sin x\), then \((f\left( {g\left( x \right)} \right){\rm{ }} = f\left( {\sin x} \right){\rm{ }} = {\rm{ }}{\left( {\sin{x}} \right)^2}\) and \(g\left( {f\left( x \right)} \right){\rm{ }} = g\left( {{x^2}} \right){\rm{ }} = {\rm{ }}\sin{x^2}\).
\(f\left( {g\left( x \right)} \right)\) and \(g\left( {f\left( x \right)} \right)\) are not equal in general, in some very special cases, they can be equal.