What does it mean that JavaScript is dynamically typed?

Direct answer

JavaScript is dynamically typed because variable bindings do not have a fixed declared type; the type belongs to the current value and can change after reassignment.

In JavaScript you do not declare a variable as an integer, string, or boolean. A binding can point to values of different types over time. For example, let value = 10; value = 'ten'; is valid.

Dynamic typing makes rapid development convenient because less type syntax is required.
It also means some type mistakes are discovered only while code executes unless tooling such as TypeScript or runtime validation is used.
Dynamic typing is different from type coercion: dynamic typing is about what values a variable can hold; coercion is about converting values during an operation.