Global settings are the 'default' settings' for Trados Studio- where you set the rules, on how Trados Studio should handle your projects and files. The 'Global Settings' could be referred to as the Engine of Trados Studio and where you can do a lot of the 'fine-tuning' to suit your requirements.
For instance, you can set the preferred 'Spell Checker' (Hunspell is the default option and is recommended by most users) or
select 'Automation'- which lets you change the general settings for your TM's