Interactivity covers a very broad canvas. At its most basic level, making something interactive simply means that a person can click a button, type in text, or in some way respond to events happening in the application that you build. Users do something in your application, and your application responds to whatever it is they do, whether it's dragging an item over top of something or pressing keys on their keyboards. There are, of course, much more complex concepts that relate to interactivity, but for most purposes, instructing your application to react to what a user does while in your application is pretty much enough.
In Macromedia Flash, the most commonly used elements added to produce interactions are buttons, movie clips, and text fields. Buttons in Flash respond to mouse and keyboard events initiated by the user. Movie clips can respond to either user-driven interactions or server-type interactions such as data loading. Text fields can be used to collect information from a user, display information for a user, or both. No matter the purpose, very nearly every Flash 8 application you produce will use all three of these items.
You do not need to write ActionScript to add interactivity, believe it or not, so you don't have to be a whiz kid programmer. Obviously, the more you know, the better equipped you are to make sophisticated applications, but Flash 8 Basic has some built-in features that you can leverage to add interactivity with very little effort (but with lots of forethought). These built-in features come in the form of behaviors, which are out-of-the box ActionScripts that you can quickly add to something, and Script Assist, a feature of the ActionScript panel used to add more complex actions without necessarily knowing everything about ActionScript.