Concurrent job runs refers to the number of jobs that can run simultaneously without queueing. You pay for compute based on the total amount of time you use compute resources, not the number of resources that you have access to. This means you can choose the right plan for your team to maximize concurrent jobs and parallelism for your jobs to minimize queuing.
For example, say you have a workflow with 10 jobs that each take 5 minutes to run. If you are limited to 1x concurrency, each of these jobs runs subsequently, and the workflow completes in 50 minutes. With scaling concurrency, you can run all 10 jobs concurrently (at the same time), and the workflow completes in 5 minutes. In both cases, your total usage time is 50 minutes since you used 10 machines for 5 minutes each. With usage-based pricing, that means you pay the same amount either way, but if you maximize your concurrency, you save 45 minutes of time waiting for your workflow to complete.