System integration
System integration is the work of making separate business systems operate as one: agreeing what each record means, moving data between them reliably, and keeping them consistent as they change.
Integration is mostly an agreement problem
The technical part — moving a record from one system to another — is usually the easier half. The harder half is deciding what a "customer", a "lead" or a "completed job" means in each system, which one is authoritative when they disagree, and what should happen to a record that exists in one place and not the other.
Why it determines whether automation works
Automation that spans two systems inherits every inconsistency between them. Where the definitions have never been reconciled, the automation will faithfully move bad data faster. This is the most common reason an automation project produces work rather than removing it.
What good looks like
A single agreed source of truth for each important record, an explicit rule for conflicts, monitoring that shows when synchronisation fails, and a person who owns that monitoring. None of that is exotic; it is simply often skipped.