Please don't worry.
I'm a primary school teacher and the parent of a child in mainstream education with special educational needs (in fact, my DC sounds quite similar to your DS).
A SENCo (or SENDCo) is a school's special educational needs coordinator. Their job is to make sure that every child is supported according to their needs so they can access learning and achieve the best possible outcomes.
If this is the first meeting you have had with school about your son's needs, you can probably expect the meeting to be attended by your child's class teacher, maybe a teaching assistant who has worked closely with him and the SENCo themselves. Your son might also be invited to join in for a bit; it is good practice to include the child or young person to get their perspective. You can ask to bring someone along with you -as you describe feeling a bit overwhelmed by it all, it can be really useful to have a second pair of ears present so you can chat about it with them afterwards.
From what you say in your opening post, it sounds to me as if your son is presenting with some challenges, and the purpose of the meeting will be to try to work out how best to support him in school. As a subtext, reading the areas of challenge you list, and the mention of the observations not amounting to a diagnosis, I wonder whether school may also want to discuss submitting a referral for an assessment by an educational psychologist (EP) with a view to refer to an assessment pathway for ASD (autism). This can feel like a big surprise (or not) for families, so they should be supportive and considerate if discussing this with you.
They are likely to open the meeting by saying something like: "We're here today to talk about what school is like for A. We have noticed that x, y and z (possibly listing areas of learning or aspects of social interaction or communication that your son finds difficult)."
Then they're likely to ask whether you have experienced any areas of difficulty at home: you can just be honest and describe what your son likes and doesn't like, is able to do and what he needs support with etc.
If they are hoping for a further assessment by an educational psychologist, they may give you some paperwork to fill in, detailing your experience of your son's difficulties at home. You can ask the SENCo for support to fill this in if it feels 'too much'. Teachers and SENDCos do forms like these all the time and know our way around the language, which, as you point out, can be a bit daunting. I would definitely say yes (give consent) to a referral for an assessment by an educational psychologist -they're hard to get.
School may want to propose setting some goals together and making a plan for how and by when they will be achieved, or at the very least, reviewed. It might be something like: "A will work with SENCo to learn two strategies for letting other children know he needs space in the classroom and in the playground. He will use these when required. A's class teacher will check in with A to make sure he does not get overwhelmed." There might be a few of these 'goals'. You should agree on a date for when you will meet again to review how it's going, and to set new goals based on how the previous ones were achieved.
It is important to remember that schools should support every child according to their needs, whether there is a diagnosis of anything, or not. So if a child struggles with learning to read, schools shouldn't hold off providing additional support, saying "This child doesn't have a diagnosis of anything yet, so let's just wait and see." Funding is short in schools across the country, and especially in special educational needs, but schools should try to pull together all the support they can reasonably offer each child, as every child and young person has a statutory right to an education -this means it is every child's right to access education.
Please don't worry, and good luck. There is a SEN board here on Mumsnet where very knowledgeable posters can help with most things.