They read from the book, from the law of God, translating to give the sense so that they understood the reading.
The Bible can be confusing sometimes. There are cultural norms that just don’t make sense without an explanation — the whole “be hospitable” adjurement, for example. There are idioms that need clarification — the whole this number (e.g. 3), even this number plus one (e.g. 4) that God feels some way (e.g. detests) about. The text is a product of its time and authors. To say otherwise is to ignore — to my peril — the influence of human beings writing anything. This does not make The Bible any less the word of God or make it any less valid, it just means that someone needs to go through and unpack and unravel the things that are cultural and will not make sense otherwise.
Recently, I’ve seen a glut of posts in social media about various hot button topics that The Bible has something to say about. My personal favorite (that is sarcasm) is the one where well-intentioned believers and hostile unbelievers argue over how much of The Law (subtitled: the ceremonial and sanitary law) should be applied to the non-Jewish believer. This, by the way, was already resolved back in Acts 15:20, 29; 21:25. Non-Jewish believers were told to avoid the pollutions of idols, strangled meat, ingesting blood (note ingestion, not transfusion, a practice not yet invented), and sexual immorality (the broad Greek word porneia which refers to all manner of sexual activity outside the bounds of heterosexual marriage). That’s it. Everything else is subject to the leading of the Holy Spirit which means there will be some tattooed, bacon-eating, mixed-cloth-wearing believers out there. And there will be others who do none of those things. I don’t think anyone argues much over the Ten Commandments and their applicability. If they do, those people need some direction.
But that’s what this verse in Nehemiah is all about: making sense of what The Bible says. Paul wrote to Timothy that he should study to show himself approved, a workman who need not be ashamed. The only way for me to know which parts of The Bible I need some help deciphering is to be studying it myself on a regular basis. Then I can be alert to those passages when I hear teachers I trust addressing them. Or I can rummage around concordances and other study tools to help understand the original words used (as above with the Greek word porneia … I don’t speak Greek). History, archaeology, there are so many fields that contribute to my understanding of The Bible that it is almost absurd. Whether I research it myself or sit under the instruction of a teacher who has been proven trustworthy, I need to make sense of what The Bible says. Not only to understand the words and meanings and context, but to be able to then apply what all of that means to me today.