The ImageJ Macro Language
What is the difference between an ImageJ Macro and Plugin ?

Plugins are usually considered being written in ImageJ's core programming language - Java. This is not 100% strict, since plugins can also be written in scripting languages, such as JavaScript, Groovy, Ruby, Beanshell, ...
If written in Java, plugins are not simple to be adapted or changed by the standard ImageJ user. If written in a scripting language, the afford is less to access and adapt a plugin.
Macros are written in ImageJ's specific macro language. This is an internal small scale scripting language that links back to Java commands inside ImageJ and is therefore a little bit slower than the actual plugins itself (but hardly noticeable).

Is is worth learning it, since it is ImageJ specific ?

You would not be here if you would not consider that already and my opinion is...  definitely YES!
If you work a lot with image analysis and ImageJ, it is a super asset, flexible and versatile enough allowing you to automate pretty much everything in ImageJ. It is super easy to learn and a good starting point to learn more complex scripting or programming languages such as Python or Java.
Especially, if you script image analyses not only for yourself but automate for your colleagues, users of your facility or even commercially for industry it serves. Advantage is that even your users or clients can later on adapt things without the absolute dependency on you.


ImageJ Macro Scripting                                   
Macros in ImageJ / Fiji, besides their relative simplicity compared to full-fledged programming or versatile scripting languages like C++, Java or Python are a very powerful tool. Macros make image editing, processing and analyses reproducible, repeatable, reduce user bias and errors while keeping error rates more constant over complete experiments. Additionally, they foster the FAIR principle in scientific data management. FAIR stands for Findability, Accessibility, Interoperability, and Reuse of data.

Macros help to:

  1. Image editing tasks can be reported and reproduced 
  2. Even small image processing steps can be speed up massively
  3. Image processing, analysis and data storing tasks can be streamlined.   
To get into macros the best start is the ImageJ Macro Recorder which can be started via Plugins > Macros > Record....  

Image Analysis Forum
The potentially most powerful resource is the actual image analysis community for the complete universe of uncountable scientific image analysis software. Since ImageJ and Fiji is one of the oldest and most used software, there are many experts around which can help you in case you get stuck with an analysis task or a specific macro.