A hotspot is simply a public birding area. It must be publicly accessible and a place where more than one birder frequents. It can be a park or refuge, a beach or wetlands--anywhere known to birders as a specific location to stop and watch birds. Note that it does not require the location to have lots of birds. In fact, it could be a place where birders stop for a single species.
All observations submitted by all observers at a hotspot are pooled, and you can view the weekly abundance and frequency bar charts for this one area just as you would for a state or county. Thus, to add to the data you would choose the hotspot off the map when submitting your list. You can then have these hotspots available in your "My locations" list of personal birding locations.
Be careful, though! If there is a hotspot for "Birder's Park," yet you create your own personal location in "Birder's Park," eBird does not combine them. This could happen if you were the first to submit a list to "Birder's Park" as a personal location, and someone later recommended it as a hotspot. Thus, if you bird a public area that is not already a hotspot, you should "recommend" the location be a hotspot when you submit your list.
If you find (by exploring data maps) that you have a personal location ("Birder's Park") but that there is also a public hotspot for the same area, you can edit and merge your checklists for that location into the hotspot. Here's how:
1) Go to "My eBird"
2) "Manage My Locations"
3) Find your location on the list of your locations and "Edit"
4) "Merge" with a location on the map
More on eBird hotspots.