Architectural pattern is the way of designing a software. You should decide which pattern to use before starting a software project. It divides the project into smaller pieces and guides you for the development.
Other definitions for architectural patterns:
An architectural pattern is a general, reusable solution to a commonly occurring problem in software architecture within a given context. (Wikipedia)
An architectural pattern is a named collection of architectural design decisions that are applicable to a recurring design problem, parameterized to account for different software development contexts in which that problem appears. (Book: Software Architecture: Foundations, Theory, and Practice)
You might come across other names like “architectural style”, “software architecture”, “development model” so on and so forth. The idea behind the architectural patterns is that solving a problem and having a common language for all of the project team (leader, analysts, developers, testers etc.)