Variable substitution is a mathematical technique used to simplify expressions or solve equations by replacing a variable with another expression or value. This method allows for easier manipulation of equations, making it simpler to find solutions or analyze relationships between variables. For example, if you have an equation involving x, you might substitute it with y to see how changes in y affect the outcome. In programming, variable substitution is often used to dynamically change values in code. By using placeholders, developers can replace these placeholders with actual values at runtime, allowing for more flexible and reusable code. This technique is common in languages like Python and JavaScript, where variables can be easily substituted to create dynamic applications.