If this option is checked, Jenkins will schedule and execute multiple builds concurrently (provided
that you have sufficient executors and incoming build requests.) This is useful on builds and test jobs that
take a long time, as each build will only contain a smaller number of changes, and the total turn-around
time decreases due to the shorter time a build request spends waiting for the previous build to complete.
It is also very useful with parameterized builds, whose individual executions are independent from each other.
For other kinds of jobs, allowing concurrent executions of multiple builds may be problematic,
for example if it assumes a monopoly on a certain resource, like database, or for jobs where you use Jenkins
as a cron replacement.
If you use a custom workspace and enable this option, all your builds will run on the same workspace,
thus unless a care is taken by your side, it'll likely to collide with each other.
Otherwise, even when they are run on the same node, Jenkins will use different workspaces to keep
them isolated.