The shape everything else depends on
Before any subsystem can be designed in detail, someone has to decide how the whole system is structured — what the major subsystems are, what each one is responsible for, and how they're allowed to talk to each other. Get that decomposition wrong and every team downstream inherits the problem, usually without realising it until integration.
What it looks like in practice
We work at the architecture level to allocate requirements to subsystems cleanly, define the interfaces between them precisely, and keep the structure coherent as the design matures rather than letting it drift into whatever shape was easiest at each individual decision point. Good architecture is largely invisible when it works — it's only obvious in hindsight, when a system that should have been painful to integrate wasn't.