A short summary and explanation of the fundamental decisions and solution strategies, that shape the system’s architecture. These include

technology decisions
decisions about the top-level decomposition of the system, e.g. usage of an architectural pattern or design pattern
decisions on how to achieve key quality goals
relevant organizational decisions, e.g. selecting a development process or delegating certain tasks to third parties.

These decisions form the cornerstones for your architecture. They are the basis for many other detailed decisions or implementation rules.