Working Memory is like your brain's task list or sticky notes. As information is coming into your brain, you are both processing it and, simultaneously, storing it. It's necessary to learn, reason, and remember.
Imagine trying to do a math problem. You need to store the numbers temporarily while you work with them to solve the problem. Keeping the information temporarily and processing it at the same time is working memory. Working memory is often used synonymously with short-term memory.