No, it's really not an inspection. Please get that idea out of your head as they are really not coming to question your religion or judge you on your home.
First and foremost, they want to get to know your child in a setting where they feel most comfortable. They want to gain lots of information about them in a setting where you feel comfortable sharing it too, away from the ears of others parents. They will already know some information about your child from their nursery but you are the expert on your child, so they want to hear it from you too. As well as needing to know about your child's needs and capabilities in certain areas (toileting, eating etc, not academics) it's also nice to find out about their interests and learn any tips to settle, distract or comfort them if they are initially upset during separation.
Home visits are especially useful for helping your child see their new teacher as a trusted adult, someone who you let into your home. They're not a stranger, they are safe with this person (or people, as the TA will usually come too). It's lovely to be able to refer back to the visit to reassure them. Do you remember when we played trains at your house and you showed me the big trampoline in your garden? I saw your cat too, he's lovely isn't he? etc. They really feel like you know them, and that's a big deal for young children in helping them to feel happy and safe in someone's care.
It's also so helpful for teachers to see the home environment, in order to help understand the family's needs better. We absolutely don't judge, but if a family are living in difficult circumstances it's important for us to know so we can support them (and also understand why the child may present as they do in school). Absolute extreme safeguarding concerns would be reported back, as a duty of care, but only in order to get them the support they need. Absolutely never to judge. This is very rare, by the way, and absolutely not what the visit is for. We are definitely not actively looking for problems to report. It most definitely isn't an inspection of the cleanliness of your home.
I'm an experienced Reception teacher and home visits are such a lovely positive experience. Yes we want to get to know your child, but we also want to build a relationship with you. The Reception year is such an important year, especially for first time parents, and it's important you feel well informed and are able to build a trusting relationship with us. What better way to start that, than with an informal chat in your own home?
Edit - with regards to what to say, just be 100% truthful when it comes to your child's strengths and weaknesses. It's important for them to know exactly what they're like. Children develop and change very quickly, so they definitely won't form an opinion on them based on what you say as they'll know that could change very quickly. Forewarned is forearmed though.