What are Barrows?
Simply put, barrows are burial mounds. They are generally not infested with wights, as in Lord of the Rings, or walking skeletons as in Skyrim. They are also much smaller than these things portray them; the Skyrim barrows look far more like catacombs than actual barrows.
West Kennet Long Barrow, in the Avebury region. There are a couple of people in this picture, but they're so small you can hardly see them. For scale, those stones at the front are all taller than a human.
Barrows usually, but not always contain burials. These can be of one person, or many people, with wealthy grave goods, or no individual markers at all.
The term 'barrow' applies right the way through from the Neolithic long barrows (above), to Bronze age round barrows (below), to Saxon barrows. That's a period of over 4000 years, so naturally they would have meant many different things in different times. 'Barrow' would not have been the builder's name for them: it's more a classification name, such as 'temple'. Synagogues, churches, mosques and so on could all be called temples, though they all mean something a little different both to believers and non-believers; the same kind of thing applies to barrows, though the different traditions were not concurrent.
Bronze age round barrows from near Stonehenge
As you can see from the two pictured examples, barrow shape and size are very variable. Many barrows were ploughed out by intensive agriculture, but those that remain are now protected, which is why these look like bright green acne in the middle of all that nice farmland.
What are they for?
Barrows were for burying the dead. In the Neolithic, that meant putting a select group of people in the tomb, generation after generation. Once the bodies had rotted a bit, you swept them back towards the sides of the rooms, and put more bodies in. In some long barrows, the bones have been carefully sorted, and in others they're a bit of a jumble. Often recognisable bones such as skulls or long bones were removed, either for reburial elsewhere, or some kind of ritual.
Bronze age barrows are a bit different. They usually had just one burial, and the monument seems to have been raised specifically to hold them. The body was often laid out ceremonially, with many grave goods. We don't really know what these goods were for; whether votive deposits (gifts for the gods), gifts for the dead, items for use in the afterlife or some other purpose. They were often in use for only one generation or so.
But barrows are big, imposing, in-your-face, right? How can a barrow be inconspicuous?
It turns out that there is another class of barrow from the Bronze age. These are usually found near normal barrow clusters, but are tiny. According to one of my lecturers, you pretty much only spot one as you drive over it in your excavation land rover.
See that tiny lump these people are standing on? That is an inconspicuous barrow.
Inconspicuous barrows are used for a much longer amount of time. Most large barrows seem to be built, used an abandoned within a few decades. The smaller barrows tend to continue to have deposits placed in them for decades or centuries.
As a tradition, they also continue to be built for longer, and are seemingly less of a thing than large barrows. By the end of the barrow-building period, many large barrow groups have been almost segregated. There are big patches of unbuilt on earth and unfarmed land around them, as if there is something taboo about their presence. Other times, people have enclosed them in small ditches, to 'fence them off' from the normal world. In contrast, inconspicuous barrows are just built around, either non threatening or ignored.
So who were these builders? The Methodists of the Bronze age world? Does this difference in barrows imply a different religious practise, with different beliefs and customs (even a different culture), or are they two traditions from the same belief system, concordant rather than competing?
To link back to the Stonehenge post, we just don't know!