In Lisp, constants are values that are fixed and unchanging during the execution of a program. They are used to represent fixed data that remains constant throughout the program's execution. Constants in Lisp have the following characteristics:

1. Immutable : Constants cannot be modified or reassigned. Once a constant is defined, its value remains the same throughout the program's execution.

2. Self-evaluating : Constants evaluate to themselves. They don't require any further computation or evaluation to determine their value.

3. Literal representation : Constants are typically represented in a literal form that directly corresponds to their value. For example, the number `42`, the symbol `foo`, or the string `"Hello, World!"` are examples of Lisp constants.

4. Common types : Lisp supports various types of constants, including:

   * Numeric constants: These include integers, floating-point numbers, ratios, and complex numbers. Examples of numeric constants are `42`, `3.14`, `1/2`, and `#C(1 2)`.

   * Symbol constants: Symbols are used to represent names and constants. They are self-evaluating and typically used as identifiers or to represent predefined values. Examples of symbol constants are `foo`, `+`, and `t`.

   * String constants: Strings represent textual data enclosed in double quotes. Examples of string constants are `"Hello"`, `"World"`, and `"Lisp"`.

   * Character constants: Characters represent individual characters, such as `#\A`, `#\b`, or `#\space`.

   * Boolean constants: Lisp has two boolean constants: `t` representing true, and `nil` representing false.

   * Other specialized constants: Lisp provides additional specialized constants for specific purposes, such as `nil` to represent the empty list or absence of a value, and `()` to represent the empty list itself.

Constants play an essential role in Lisp programs, providing fixed data that remains constant and is used in computations, comparisons, and other operations. They provide a way to represent and work with unchanging values, making code more expressive and allowing for efficient execution and optimization.