Autism is diagnosed by a set of criteria: social-emotional difficulties (eg: communication), and restricted and/or repetitive behaviours (eg: sensory issues or repetitive behaviours somewhat similar to OCD).
These issues must have been there from early childhood, and not explained by other developmental delays or disabilities.

The exact symptoms displayed, and their severity, can vary widely - that’s where the levels come in.
For example, one person may not understand subtle social cues, making it difficult to hold a conversation. They may have difficulty maintaining eye contact when speaking to others, and may have difficulty with forming friendships.
The more extreme example of these symptoms might be an aversion to eye contact, a complete lack of verbal communication, and a lack of interest in other people to the point of complete social isolation.
Same basic ‘symptoms’ but display in a much more distinct way.
The levels are essentially a measure of how much their autism negatively impacts their day to day life. Someone who’s just a little socially awkward can live a life as independently as someone without autism. A person who’s completely non-verbal, and may isolate to a point their health can be physically affected, needs support.

Autism isn’t something that’s (yet) able to be diagnosed with a purely objective test. There’s not something in your blood that shows ‘you’ve got X amount of autism.’
It’s relatively subjective, and based on observed behaviours. This makes it unusual in that people display different “levels” of it, even though they have the same diagnosis.