What are SQL Server Securables?
A securable is anything that can have permissions granted, denied, or revoked on in SQL Server.  It's basically "something you can secure".

Think about it like this:  Your car is a securable.  You would have to have been given keys (permissions) to access your car (securable).

A securable is anything that can be accessible or inaccessible.

Take a look at this BOL reference on Securables.  Under the three scopes of securables (server, database, schema) you can see a listing of the contained ones.  Here is the list of securables (taken directly from BOL, but for reference):


  Server Scope Securables
  
  
  Endpoint
  Login
  Database
  
  
  Database Scope Securables
  
  
  User
  Role
  Application role
  Assembly
  Message Type
  Route
  Service
  Remote Service Binding
  Fulltext Catalog
  Certificate
  Asymmetric Key
  Symmetric Key
  Contract
  Schema
  
  
  Schema Scope Securables
  
  
  Type
  XML Schema Collection
  Object
  
  Aggregate
  Function
  Procedure
  Queue
  Synonym
  Table
  View