The Highlands are an area of over 40,000 square kilometres. Sure, on a map the area all looks the same but check out this article on the top 5 places, you'll see that they are all different https://guidetoiceland.is/best-of-iceland/top-5-places-to-visit-in-the-highlands-of-iceland
F-roads are the mountain roads of the Icelandic highlands. They can be a bit tricky navigating if you are only used to driving on paved roads. They are usually quite narrow, so you shouldn't drive fast on them.
Depending on weather conditions the roads can be open or closed, this is usually due to snow. If there is too much, the roads will be closed. Iceland has a road administration that determines if the roads are open or closed. You can check their website to see which roads are open on http://www.road.is/
These F-roads have a surface of loose gravel. The same applies to the highway roads as well, although they have long stretches of asphalt. When you drive these loose gravel roads, you should always be careful and slow down whenever approaching an oncoming car.
The best vehicle for driving these F-roads are 4WD jeep/SUVs. And for Icelandic conditions, I would choose a manual over an automatic. If you rent a car, it usually comes with a GPS.
PLB could help if you are hiking the Icelandic highlands, or travelling in the winter. Here is the information about PLB on the Icelandic Associaton for Search and Rescue http://safetravel.is/rent-a-plb/
They do GREAT work by the way. If you get lost and have to use your PLB, they are the ones that will come and rescue you. They'll save you even without the PLB, but they'd have to search first, so on their website, you can fill out a form where you tell them your travel plan, so they know where to search.
The Search and Rescue team is all made up of volunteers so if you get a chance to support them financially you should do that (they sell cute keychains you can buy).