Here you need to allocate space for storing Linux. Minimally it needs a partition of several Megabytes, we recommend several partitions and 2GB in total for end user system (i.e., "Workstation" install option) for data safety, smoothness of system running and ease of backup.
There is no general rules of partitioning that fit all situations. Plan according to your specific requirements.
There are three choices of disk partitioning:
Pick the first one only when you want an easy life. All disk space not used in any partition plus any existing Linux partitions will be used to create new pre-set linux partitions. All data in the existing Linux partitions will be removed. No recovery is possible.
The next two choices requires you to have knowledge on disk partitioning. Both of them are not difficult, but partitioning with Disk Druid is a bit easier than with fdisk, so we go for the second choice - Disk Druid. (But sooner or later, you need to use fdisk.)