Embedded forms is a simple form editor, where you can quickly create sign up forms for your contact lists. You can customize the form or use it "naked" without any styling, which you can then insert into any website you have.
In order to create a new embed form, navigate to the list you want to add a form for under the menu "Lists". In the sub-menu click the "Forms" link.
At the right side you will see the "Form generator", where you can enter the editor.
You can see how the editor is used in the video below:
Once done, copy the embed script and insert it on any website you want.