The use of the API lets you communicate with your JW Player Dashboard, allowing you to manage your videos, playlists, player and more directly in WordPress.
How to get my JW Player API credentials
Access to your JW Player Dashboard and, from the homepage, click on Api Keys up on the right.
Click then on SHOW CREDENTIALS and get your API Key and Secret.
Setting the plugin
Now you have just do add the API Key and Secret in the in the specific fields that you can find in the plugin options page.
If you can't see them, what you have to do is just add a cloud player url first, since the plugin changes the options available based on which kind of player you're using, self hosted or from the cloud.
Here more information about how to configure the player.