What is the Subtitle?
Its weight for the search algorithm is lower than the weight of the title. But it is higher than the weight of the keyword field or IAP titles. So apps with a keyword in the subtitle will, in general, outrank apps with the same term in the keyword field or in an IAP title.
The subtitle appears in most conversion opportunities: It will show up in SERPs, in top charts, and also in most features. Thus, it has an enormous potential to increase the conversion rate. But in some placements, it is not visible permanently. Instead, it rotates together with the developer name.
It is common to name the core features of the app in the subtitle. There is no need to write complete, grammatically correct phrases. So simple lists of comma-separated terms are fine. Have a look at the example from the app 8fit below.