If you check the rows in the "k2_extra_fields" table, you will find the definitions for the extra fields.

I can see that this is supposed to be an array of objects:
[{"name":null,"value":"","target":null}]

The first two are quite explanatory, but what is the last property, "target"?

I'm pretty sure this is for "weblink" extra fields.
if your extra field is a weblink you can assign the target.