The JSTL <c:set> tag is used to set a value for a variable or attribute in one of the JSP scopes, such as the request or session scope. This tag is useful for setting values that can be used later in the page or in other pages.

The basic syntax for the <c:set> tag is as follows:

<c:set var="variableName" value="variableValue" scope="scopeName" />

The var attribute is used to specify the name of the variable or attribute to be set, while the value attribute is used to specify the value to be assigned to the variable or attribute. The scope attribute is optional, and if it is not specified, the variable or attribute will be set in the page scope.

Here's an example that shows how to use the <c:set> tag to set a variable in the request scope:

<c:set var="myVariable" value="Hello, World!" scope="request" />

In this example, the myVariable variable will be set to the value "Hello, World!" in the request scope.

Note that the <c:set> tag can also be used to set the value of a property on an object, using the property attribute instead of the var attribute. For example:

<c:set target="${myObject}" property="myProperty" value="myValue" />

In this example, the value of the myProperty property on the myObject object will be set to "myValue".

The <c:set> tag is a powerful tool for setting values in the JSP scopes, but it should be used judiciously to avoid cluttering the scopes with unnecessary variables or attributes.