What it means
A single-threaded deal runs through one person, usually the contact who first took the meeting. A multi-threaded deal has several working relationships on the buyer's side: the champion, the economic buyer, the people who will use the product, and whoever handles technical, security or procurement review. Each thread gives you a different view of the deal and a different way to keep it moving. Multi-threading isn't copying everyone on every email. Each contact should hear about what matters to them.
Example
Six weeks into a deal, a rep realizes every conversation has been with one operations manager. He asks her who else the change will affect and offers to run a short session on security for her IT lead. In that session, the IT lead mentions a review step the operations manager didn't know about, and the rep adds it to the plan.
Why it matters
Single-threaded deals are fragile. If your one contact changes jobs, goes on leave or loses an internal argument, the deal stalls and you may not even know why. The same risk applies to existing customers, where a renewal can be lost because the person who bought the product has left. Several threads also improve the deal itself, because different people raise different requirements and objections early, while there's still time to address them.
How to practice it
The hard part is asking your main contact to introduce you to others without making them feel bypassed. Practice framing it as help for them: 'Your finance lead will probably ask about the cost case. Would it help if I walked her through it, so you're not answering every question alone?' Then practice short, separate conversations with each new person, focused on their concerns rather than a repeat of the full pitch.