Autism exists on a multi-dimensional spectrum. Dimensions include restricted interests, anxiety, social awkwardness, difficulty reading faces, sensory processing issues, unusual motor of verbal behaviour, among many others. You can think of these characteristics among all people as being distributed like bell curves, and we use the word autism to describe people who are on the tail ends of these. This can also be described as "having autistic traits".
These characteristics tend to be correlated, as well as with neurological differences. But it's important to remember that they are blurry distributions, and also that every autistic person will lie in different points along these characteristics. It's even somewhat conceivable for two people to be autistic and share no autistic traits.
Autism is not by itself a pathology. However, if an autistic trait is especially pronounced, it might cause clinically significant impediment to a person's ability to take care of themselves or integrate with society (especially modern society — more on that later). We can then speak of ASD, or Autism Spectrum Disorder, which is a clinically recognised diagnosis. Many people with ASD also have a learning disability, but certainly not all.
While many difficulties autistic people face are intrinsic to autism, a significant proportion also come from stigma. Modern society is conformist, especially in the working world, and the natural needs, behaviours, and mannerisms of autistic people can conflict with expectations of professionalism or convention. So, high-functioning autistic people have a tendency to consciously or unconsciously learn to imitate neurotypical behaviour, sometimes at great expense in terms of energy and self-identity. This is called "masking".
It's a matter of definition whether we use the word "autism" to refer to this cluster of traits or the clinically significant disorder. Indeed, I think the word used to refer exclusively to the disorder, and it remains controversial. Many people, especially carers of people with low-functioning ASD, feel that saying autism is not an illness trivialises the genuine difficulties they experience. I am very sympathetic to this, but ultimately, I think that without this word "autism", many otherwise fully independently functioning people who fall under the umbrella are left with a void to explain their feeling of difference and social exclusion, and it is bad for society as well, as it pushes autistic people to suppress traits that could potentially be extremely valuable. So, I advocate for this autism vs ASD distinction. But reasonable people can disagree.
I hope you found this useful as a primer on autism, or at least of this autistic person's understanding of it!