A surd is a radical that is not evaluated, or cannot be precisely evaluated. The radicand is often a constant, such as the square root of two: We know that the square root of 2 is 1.4142.. But why do we leave it as a radical and not convert it to the number? There are a couple of reasons:

It may cancel later

As you are working through a problem, you may get something like: The surds on the top and bottom cancel leaving

It may get raised to a power later

As you work a problem, you may, for example, end up squaring the surd: Since root 2 squared is obviously 2, the expression simplifies to If we had replaced root 2 with 1.414 earlier, we may not notice that squaring it results in exactly 2.

It preserves accuracy

Some radical expressions result in irrational numbers, with a result that has an infinite number of decimal places. For example, the square root of 2 is irrational. No matter how many decimal places you use in the answer, you can always add more and make it more accurate. We try to leave it as a surd, so so we are not pinning down the accuracy until later when we finally do evaluate it.