The startup flags or startup command are the instructions you give the server on how to boot your game. Game developers sometimes allow for you to changes setting by using then startup command, or for games like Minecraft you can sometimes improve performance by selecting the correct settings.
For most games that are not Minecraft. You can change any startup settings in the quick settings options on the overview page.
For Minecraft (Java), you'll need to select flags with the startup flags drop down.
Selecting The Correct Flags (Minecraft Java)
No matter what you're running we recommend you select the Aikar flags. These are the most optimised settings for Minecraft and can help with memory crash issues, and allow your server to run smoother.
You can select these flags in the drop down. Make sure you select the correct option for your RAM value. Otherwise you will get memory crash issues.