A .jl file is a source code file written in the Julia programming language. Julia is a high-level, high-performance dynamic programming language designed for numerical and scientific computing. It is known for its speed, which is comparable to statically-typed languages like C and Fortran, while maintaining the ease of use and flexibility of dynamic languages like Python and R. Julia code is typically used for tasks such as data analysis, machine learning, simulations, and mathematical modeling. The .jl file contains human-readable code that is compiled or interpreted by the Julia runtime environment to execute the program. These files can be opened and edited with any text editor, but specialized IDEs (Integrated Development Environments) offer features like syntax highlighting, debugging, and code completion to improve the development experience. Julia's design emphasizes performance and parallelism, making it well-suited for computationally intensive tasks.