Hi there!
If you do the command /ms or /milestones in-game, you can see two types of milestones; personal and island milestones. Completing the island milestones are the things that will help you reach new island levels, depending on which you complete!
To see what milestone you need to complete in order to gain the next island level you can type the command /is. After this you right click the island you want to see it of and then you click 'Island Levels' (the ladders). Every level will have a certain amount of milestones to complete and sometimes some other requirements!
For more information you can check out the in-depth guide to Skyblock on this forums right here:
[Skyblock] An in-depth guide to Complex Skyblock
Hope this helps!
