Project specifications are an incredibly valuable guide for your work. They’re usually a text document that specifies the goals and requirements for a project. Specifications can come in various formats with a variety of content. They might not even have the name specification. Sometimes they're called requirements briefs or project guidelines.
These documents could be part of a contract formalized as tasks in project management software or just a list in an email. A specification can be instrumental in helping you manage goals and expectations with your clients and managers. They won't be provided for you as a designer. You'll often be the design expert in the room, and you'll have a better idea of common opportunities and pitfalls in similar design projects.
You’ll rarely come across a project where you get all the necessary information provided from the start. You should also ask lots of questions about the project to help you understand what’s needed from you. Learning the expectations and the needs of the people you're working with can help you create great work that satisfies everybody.
And even if you’re working on a personal project, creating a specification can help make your own requirements clear and give you something to refer back to while you work.