Post Main Marker is a marker that will be displayed on global map (this is a map on homepage).
Setting Main Marker
You can set that marker in various ways. The easiest would be to just drag and drop it to the place you want. Other way is to type address of the place you want to select. Thanks to autocomplete feature, it will use the same way of autocompleting addresses as the Google Maps use.
After setting marker position you can set up marker’s infobox. You can select any image (if you won’t, the Featured Image set in post will be used), change the icons color using ColorPicker, or set your short custom text for the marker (if you won’t, it will show first 16 words from the post content)