In the Template Editor, Safezones are a really useful tool. When selected for a pre-defined template they will outline the zone for that particular template (Youtube, TikTok etc.) that is 'safe', i.e. will not be covered by other elements inherent to that template.

For example, here is a 1:1 template. If I apply the 'Youtube square' safezone you will see in the second screenshot it has overlaid the area that is usable in square Youtube content,

As you can see the safezone, or safe usable area, is quite a lot smaller and a different shape to the original 1:1 ad.

In this 16:9 ad, we see a similarly unpredictable are when we select 'Youtube Horizontal',

Hopefully you can see the power of safezones - they take a lot of the work out of working out where you should place your various ad elements.