In Sri-speak, a concrete deliverable is something that can be seen or held. For example, a logo design is concrete once you can see it on a screen or print it on a piece of paper that you can hold. A piece of music is concrete once it is in an audio file format or pressed onto a CD master.
When assessing productivity, I will look at the number of "concrete elements" I have produced. This keeps me from engaging in work that might not be productive, like an excessive amount of researching without making a prototype, or spending a lot of time talking about something but not doing it.
When working with other people, I make the concrete deliverable the centerpiece of my project scoping. There's two reasons for this:
When working with people who are primarily consumers, they like to get something for their money that "feels real". They aren't really interested in the time and effort it took, and assess the value of the work based on what they can do with it to enhance some aspect of their life.
When working with people who are business practitioners, there is recognition that the time to create and deliver has an expected cost because they have to deal with this too. They still have the expectation that they will receive some benefit or value as consumers do. If the business client is not familiar with how the benefits work because they are not specialists, the expected value may not be clear to them, which is a dangerous condition in a contract. For these clients, focusing on concrete deliverables can help during project scoping.