It just means that Excel is utilizing n number of cores to do its number crunching, simultaneously. It's like Excel is distributing the work among the multiple cores, to be done concurrently / in parallel, so as to be faster. To give you an analogy, when you go to a restaurant with your family, and you make a large order, the kitchen might spread your work among multiple cooks, instead of a single cook cooking one dish, then the next, then the next, sequentially. They don't want you to wait an hour, so they will distribute the cooking tasks among multiple cooks
Same principle here.
It must be using the word "processor" to mean "processing core" or "core", since you don't have more than 1 processor, you have more than one core.
Are you using the same version of Excel at home and work ? I ask because, older versions may not be as adept at utilizing multiple cores. This goes for older software in general. Any piece of software has to be "aware" of multiple cores to be able to take advantage of and harness that extra processing power. That may explain why your home Excel only uses 1 core, while the one at work uses 8 simultaneously. Now, processors with multiple cores have been around for quite a while, so I'd be shocked if Excel doesn't know how to take advantage of them. And clearly, the Excel you have installed at work does.
Or, it could just be that the computations you happened to be doing at home were not as CPU-intensive (that is to say, requiring a lot of CPU work) as the computations you were doing at work. So, at home, Excel needed only 1 core, while at work, you were really stressing Excel to the limit and it had to use 8 cores to crunch numbers.
Another possibility is that, at home, you had a different workload (different set of apps running simultaneously), so the number of cores actually "available" to Excel may differ from the number of cores your work machine was able to provide. It depends on the total system workload at the time when you're using Excel. So, at home, maybe you were running a lot of apps, and the system was not able to "give" Excel multiple cores at that time. This goes into the complex subject of how an operating system performs thread scheduling (a thread is an indivisible/atomic unit of process execution).
To summarize, the number of cores a program will actually use at any given time depends on:
1 - Whether the program is actually "smart" enough to know how to use multiple cores.
2 - What kind and amount of work/computations the program performs (number crunching vs file duplication, etc)
3 - The operating system's thread scheduling algorithm(s)
4 - The workload (running processes) on the system at that time
