A .pkl file, often referred to as a "pickle file," is a file format used in Python for serializing and de-serializing Python object structures. Serialization is the process of converting a Python object (like a list, dictionary, or even a custom class instance) into a byte stream that can be stored in a file or transmitted over a network. De-serialization is the reverse process, reconstructing the Python object from the byte stream. The pickle module in Python provides the functionality to perform these operations. Pickle files are useful for saving the state of a program, caching results of expensive computations, and transferring complex data structures between Python applications. However, it's crucial to understand that pickle files are inherently insecure if loaded from untrusted sources. The pickle format allows for arbitrary code execution during de-serialization, meaning a malicious pickle file could potentially compromise your system. Therefore, only load pickle files from sources you trust completely. Alternatives like JSON or YAML are generally preferred for data exchange with external systems due to their security and interoperability.