A radical expression is an expression that contains a square root symbol (√). The number or expression inside the square root symbol is called the radicand. The square root symbol tells us to find the number that when multiplied by itself, gives us the radicand.

For example, the expression √9 means "find the number that when multiplied by itself, gives us 9". The answer is 3, because 3 * 3 = 9.

Sometimes, the radicand is a more complex expression, such as √(x + 4), which means "find the square root of (x + 4)". To simplify the expression, we need to figure out what number, when multiplied by itself, gives us (x + 4).

Radical expressions can also contain a number outside the square root symbol, like 2√9, which means "2 times the square root of 9". The square root of 9 is 3, so 2√9 means 2 * 3 = 6.

Radical expressions can be used in many real-world situations, such as finding the length of a side of a square when we know the area, finding the distance between two points, or finding the height of a tree. They are important to understand in mathematics and science.