The surd form of $\sqrt{27}$ is $3\sqrt{3}$.

To understand why $\sqrt{27}$ can be expressed as $3\sqrt{3}$, we first need to factor the number $27$ into its prime components. We can express $27$ as $3 \times 3 \times 3$, which can also be written as $3^3$. When we take the square root of $27$, we are essentially calculating the square root of $3^3$.

The square root of a product can be separated into the product of the square roots. Thus, we can rewrite $\sqrt{27}$ as $\sqrt{3 \times 3 \times 3}$. By grouping the factors into pairs, we simplify this to $\sqrt{3^2 \times 3}$. The square root of $3^2$ is simply $3$, since the square root and the square cancel each other out. This simplification results in $3\sqrt{3}$.

In surd form, we retain the square root of $3$ as it is because it is an irrational number that cannot be simplified further. Therefore, the surd form of $\sqrt{27}$ is $3\sqrt{3}$.

This surd form is particularly useful in various mathematical contexts, especially in algebra and geometry, as it provides exact values and helps avoid the approximation errors that can arise from decimal representations.