It is all about Keys and Access
A SharePoint Farm is like a Business Park:
then Site Collections are Buildings:
As Site Collection Owner you have a MasterKey to all information in this building.
Permission Profiles can be centrally managed, *but alas most SharePoint 'consultants' don't use its power
Subsites (Webs) are Rooms:
Document Libraries/Lists are Cabinets:
Documents-Sets/Folders are File Suspension Holders:
DocumentSets are available since SP2010 and underestimated by many
(and most often completely unknown)
and Documents/List Items are Files:
Permissions to all
On each of the above you can have different locks(permission profiles) (each can have 33 types of SharePoint permissions to be precise).
And you have to check each one to see what permissions you have
Either by hand or a Powershell script.
Search can give you a clue where you have Read rights (but you can have Write rights without Read rights..)
And then the metaphore goes wrong...
OMG! In the digital world you can have access to a File and NOT have access to the File-Holder or Cabinet or Room or Building or Business Park.
Most will try and sell you 3rd party tools to manage this all... but if your foundation is crap, no tool can fix it for you
Just like an architect plans every detail of a building,
good Permissions planning for sites and content in SharePoint is key