When we say "straightforward," we mean that the sentences move ahead with a pretty standard "Noun + Verb" structure:
The two rabbits went up to the board at a hopping run and crouched in a patch of nettles on the far side, wrinkling their noses at the smell of a dead cigarette end somewhere in the grass. Suddenly Fiver shivered and cowered down. (1.26)
That first sentence isn't small and simple, but each segment of it moves the action forward or adds to the description. You can read an Adams sentence as answering a set of questions:
What did the rabbits do? They went up to the board. And then? They crouched. What did they find there and how did they respond? They wrinkled their noses at cigarettes (which are one of those human things that rabbits don't like).
So it's a long sentence, but a pretty straightforward one, nonetheless.
Then we hear about Fiver's reaction and learn more about him: he reacts "Suddenly" (because he's jumpy and nervous); not only does he shiver, but he cowers (because he's extra nervous); and no one ever "cowers UP," so saying "cowered down" is a bit redundant but it reminds us how nervous Fiver is.
So remember when we said that the style here was "Straightforward (Mostly)"? This is the "mostly" part. Occasionally, the sentences will keep that "Noun + Verb" form that we love so much, but slow down the action to describe something. Sometimes that description may be in the form of repeating the same basic idea to emphasize how much this is true. This short sentence about Fiver really just wants to describe how nervous he is.
And sometimes you have to pay super close attention because that description might only be a word or two. For instance, notice that the cigarette that they smell isn't "extinguished" or "used"—it's "dead," a word which carries a slight hint of danger.
We need a whole special (but short, we promise) subsection on the description of nature in this book. Because there's a lot of it. Like seriously, this book could be used as nature guide based on the sheer number of flowers it mentions. For instance, the first chapter goes on for about three paragraphs about the flowers, the grass, the sunset, and what all the animals are doing. (Hint: not their taxes.) And then eventually we get to hear about the rabbit main characters and the rabbit society.
Or, for another example, take the long paragraph in chapter 28 which is all about Watership Down in the moonlight. It's a whole paragraph describing the landscape and how it looks different at night. Here's a small taste:
In moonlight, two acres of coarse bent grass, undulant and ankle deep, tumbled and rough as a horse's mane, appear like a bay of waves, all shadowy troughs and hollows. The growth is so thick and matted that even the wind does not move it, but it is the moonlight that seems to confer stillness upon it. We do not take moonlight for granted. It is like snow, or like the dew on a July morning. It does not reveal but changes what it covers. (22.28)
Check it out: it's a whole section that doesn't have to do with plot or with character. It's pretty slow and maybe requires a little extra care because it's written in a different style. For instance, notice the long pause between the noun and the verb of that first sentence: "two acres of coarse bent grass [noun] […] appear [verb]." Everything in between that noun and that verb is just description.
All this long paragraph does is describe the natural landscape—and, also, talk about our relationship to that natural landscape. Here, the moonlight may remind us that the Downs aren't just good land for building or hunting. This long description of the moonlight reminds us that the natural world can be enjoyed just as it is. As long as we don't mess it up too badly with cigarettes and apartment complexes.