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.

As a minimum, you should set your Language Pairs

Create (or use an existing) a TM (Translation Memory)

Create (or use an existing) TB (Termbase)

For instance, you can set the preferred 'Spell Checker' (Hunspell is the default option and is preferred by most users).

You can also fine tune your TM...

TIPS