Indexing and availability stipulates where people will discover your work. You want to submit your work to journals that people in your field will easily find through keyword searches within databases. There are two simple ways to find out where journals are discoverable.
The first is to go to a journal of interest's website and click on About this Journal, where you will find some version of this information. Examples:
Another way is to go to a database you know people in your field often use and find the list of journals they index (they may index an entire article or just the abstract and keywords).
Here is an example of performing that search:
1. Find the Browse by Subject area in a database
2. Select a subject listing best suited for your manuscript
3. Explore the list of journals available in that subject area within that database