The Binary Data plugin allows raw access to an allocated section of memory (often referred to as a buffer). For example it could allocate 16 bytes of memory, and read and write anywhere in that buffer as individual bytes, 32-bit integers, floating point numbers, text, and so on.
A comprehensive description of how binary data/computer memory storage works is out of the scope of this manual. However there is lots of information on the Internet that covers it, and most computer science or computing courses will also cover it. The Binary Data object is also useful even if you do not access its contents: it integrates with other plugins like AJAX and Local Storage, allowing binary data such as images to be stored or transferred in useful ways. Despite the name, Binary Data can also store text, using the Set from text action and GetAllText expression, which can be useful with other plugins such as Cryptography.