I must say that if you must ask the question, you don't need to know...but I know you just came through an interview and you are trying to gain knowledge...so I'll give it to you....but it's a bit more advanced than you currently are.
A container, in Remedy terms is an object that contains references to other objects. 3 examples of containers would be an Application, an active link guide, and a filter guide. Containers contain pointers to other objects that are stored in the arreference table....so, if you build an active link guide and add 4 active links to it, you aren't adding a copy of the active link, you are adding a reference to the active link....from a remedy developer perspective, the difference is irrelevant...but when you are dealing with backend tables, or through the api, it's important to know the difference and how to identify the types of containers and types of references so that you can reconstruct things properly.
Refer below documentation link for details.
Sr. Application Analyst