One-piece flow (or single-piece flow) is one of the core concepts of lean manufacturing.  The idea is that we should produce one item at a time, when the customer needs it. This concept stands in opposition to batch-and-queue production, in which we make a whole bunch of one thing, then make a bunch of the next thing, etc. A lean car factory might make one green Corolla, then one blue Camry, then one red Yaris, for example, while a mass production factory would make 100 black Wranglers, then 100 purple Cherokees. For many reasons, it's way more efficient to run the factory the first way rather than the second.

But while batch-and-queue production isn't the best way to run a factory, in many respects it is the best way for you to work. Why? Because unlike the single-task machines on a production line, you perform many types of operations: talking on the phone, writing emails, building spreadsheets, reviewing proposals, solving a problem with one of your colleagues, dunking Krispy Kremes in your latte, etc. The need (and ability) to do so many types of things makes you a "monument machine."

Monument machines are fantastically complex, terrifically expensive, and perform an amazing variety of tasks. (There. I've used up my superlatives for the week.) Manufacturing engineers love them, because they're so cool. Lean adherents hate them because they necessarily require non-productive changeover time when they're switched from one task to the next. Lean companies are always looking for ways to get rid of them and replace them with simple, single-task machines. But when they can't be eliminated, it's most efficient to do batch production: first paint all the red cars, then all the green cars, then all the black cars, for example.

Just like a monument machine, there's non-productive changeover time when you switch from one task to the next. When you stop working on a spreadsheet to answer an email, it takes your brain a little while to transition to the processes required for the new work.  David Meyer, a psychologist at the Federal Aviation Administration, explains that

In effect you've got writer's block briefly as you go from one task to another. You've got to (a) want to switch tasks, you've got to (b) make the switch and then you've got to (c) get warmed back up on what you're doing.

While your brain is pretty damn fast at this transition, there is a real time cost.  Meyer says that

People in a work setting who are banging away on word processors at the same time they have to answer phones and talk to their co-workers or bosses -- they're doing switches all the time. Not being able to concentrate for, say, tens of minutes at a time, may mean it's costing a company as much as 20 to 40 percent in terms of potential efficiency lost, or the "time cost" of switching, as these researchers call it.
And there you have it: pure muda. Pure waste.

My prescription for reducing this waste is to batch process your various types of work.  If you need to have 1:1 meetings with people, do them in one batch. If you have to work on spreadsheets or write code, do that in one batch. Whatever you do, don't interrupt these tasks by checking email. Instead, batch process your email a few times per day. I promise you won't miss much: if there's something really important, the sender will find you. Really. (But do them (and yourself) a favor  -- let them know in advance that you're going to a new mode of email processing.)

One of my clients doesn't allow me to use the word "batch" when I teach their staff, because it doesn't fit with lean principles. They prefer the vaguer, more euphemistic injunction to "group similar types of work." But I believe that batching isn't a dirty word when it reduces the waste in a process. And when you're dealing with human monument machines, it most certainly does.