Disclaimer: I am most certainly not an expert and am just posting my understanding based on other Aspies I know and posts on here by users I respect.
Functional levels aren't perfectly related to intelligence, but there is something of a relationship.
Someone with low functioning autism probably can't do many "routine" tasks for themselves, such as washing or getting dressed (they might be able to do some of them, or they might require lots of prompting). They struggle to hold normal relationships and require a lot of slack. They will probably never be able to hold down a traditional job or even get through interviews. However, they may be intelligent and capable of expressing themselves, particularly with the held of computers.
Someone with "medium" functioning autism can probably do those tasks to a degree, though they might require prompting and might struggle with more complex tasks (like cooking). They might be able to get a job, but statistically most won't last long. They might be able to have something that resembles a friendship between NTs with other people, but will probably require more tolerance than would be afforded to an NT.
Someone with high functioning autism can probably do most routine tasks, although may require prompting on occasion (e.g. if their routine is disrupted). They might well be able to manage more complex tasks like cookery and housework. They will often pass as NT to the untrained eye, though they will often be considered somewhat eccentric or odd. Many can hold down friendships and even long term romantic relationships. Many will hold down jobs, many will be in and out of work a lot.
Functional levels can probably vary to an extent over time e.g. with the acquisition of new skills, or they could fall due to mental illnesses like depression, or stress.