This is the most simple explanation about Dependency Injection and Dependency Injection Container I have ever seen:

Without Dependency Injection

• Application needs Foo (e.g. a controller), so:
• Application creates Foo
• Application calls Foo
  • Foo needs Bar (e.g. a service), so:
  • Foo creates Bar
  • Foo calls Bar
    • Bar needs Bim (a service, a repository, …), so:
    • Bar creates Bim
    • Bar does something

With Dependency Injection

• Application needs Foo, which needs Bar, which needs Bim, so:
• Application creates Bim
• Application creates Bar and gives it Bim
• Application creates Foo and gives it Bar
• Application calls Foo
  • Foo calls Bar
    • Bar does something

Using a Dependency Injection Container

• Application needs Foo so:
• Application gets Foo from the Container, so:
  • Container creates Bim
  • Container creates Bar and gives it Bim
  • Container creates Foo and gives it Bar
• Application calls Foo
  • Foo calls Bar
    • Bar does something

Dependency Injection and dependency Injection Containers are different things:

• Dependency Injection is a method for writing better code
• a DI Container is a tool to help injecting dependencies

You don't need a container to do dependency injection. However a container can help you.