Sometimes the Install Ubuntu alongside Windows Boot Manager option in the Installation type screen of the Ubuntu installer is the way that the Ubuntu installer incorrectly identified Windows, because that is the way that the existing Windows OS is identified in the system's configuration.
Is it safe to choose that option?
It's not always safe to choose that option. If there is already a Windows EFI system partition, the Ubuntu installer will often detect it and use the existing Windows system partition instead of creating a new one, but
if the Ubuntu installer did not correctly identify the existing Windows OS it's possible that it also did not identify the partition that is used by the computer to boot Windows, if such a partition exists. In such a case it's possible that Ubuntu will not install the grub bootloader because it doesn't know where to install grub, and the entire Ubuntu installation will abort. If this happens you can still install Ubuntu by choosing the Something else option at the Installation type screen of the Ubuntu installer.
The EFI System Partition (ESP) is a partition on a data storage device (usually an HDD or SSD) that is used by computers adhering to the Unified Extensible Firmware Interface (UEFI). The EFI System Partition is an interface that's used by the computer to boot Windows. It's like a step that is taken before it runs the Windows partition. It's a small partition, but without that partition your computer wouldn't know how to boot Windows.
The EFI System Partition is a dedicated partition on GPT. It's usually a small one (100-500 MB) formatted as FAT located at the beginning of the disk, and its partition record is at the beginning of the GPT (GUID Partition Table).