The story is set in the far future where humanity has spread out across the galaxy living on other worlds & meeting other intelligent species. All the action is set on other planets, and mostly on Arieka, the planet where Embassytown is. The story is told by Avice, who grew up in Embassytown but left to travel and crew on spaceships. The first half of the book is split alternating between her present when she's returned to Arieka and flashbacks to her upbringing and youth. And then once the past catches up to the present it moves on as a single narrative (and this structure is probably symbolic of the structure of the story too).
Humans are not the only intelligent lifeforms on Arieka - the Hosts are the alien species who evolved there. They speak Language, forming words simultaneously with their two mouths. Somehow the structure of the language & of their thoughts means that it is not possible for them to lie. They also in some way need the presence of a single sentience behind the words to be able to comprehend it as Language, so the only humans that can speak to them are pairs of identical clones trained from birth to form the two vocalisations of Language with a single thought behind them. These Ambassadors also wear links, electronic devices that enhanced their natural empathy. And when they speak human languages to humans they complete each other's sentences - they are as much as possible one mind in two bodies.
One of the themes of the book is unintended consequences. The planet from which Arieka was colonised have sent their own Ambassador to talk to the Hosts, who is not a pair of identical clones and this has world shattering consequences. Avice comes home and brings her current husband to see where she grew up - he's a linguist and fascinated by Language. His presence during the crisis also has repercussions that could not have been expected. Also humans just by existing on the planet have changed how the Hosts behave and think.
Another theme is that the language we speak affects the way we think. Most obviously in the Hosts, whose Language doesn't permit them to lie - and they have to create actual real things to use as similes. So Avice acted out something for the Hosts when she was a child and she is immortalised in Language as "There was a human girl who in pain ate what was given her in an old room built for eating in which eating had not happened for a while", shortened over time to "the girl who ate what was given her". So then the Hosts can refer to something as "like the girl who ate what was given her". And the plot hinges round this nature of the Hosts & their Language in a satisfying way - it is both integral to the crisis and to the resolution of it.
Those similes are obviously translations, and another theme that runs through the book is how translation hides meaning. You turn the words of one language into the words of another, and you think & hope that you now understand what the first one was saying. But you don't necessarily, meanings can be lost and changed by the act of translation. And having turned it into words you do understand you don't notice any of the lost things any more because you think you understand it. Even if you're speaking the same language you bring the baggage of your past experiences along with it - there's a bit near the end where Avice reflects that Scile (her husband) wouldn't've done or said the things he did if he'd been from Arieka or another more recent colony because he would've known what it meant.
I definitely enjoyed reading this on a surface level - the story carried me along, I sympathised with the characters who I was supposed to etc. But I have a feeling there was a lot more going on underneath the surface than I really got out of it. Perhaps partly because I did have a longish break in the middle of reading it.