It means the text will be difficult to understand for a "layperson" (someone who does not know a lot about the subject of the text). It will contain uncommon (topic-specific) words, and complex explanations.
"Pretty" means "somewhat" here. This is a common usage in US English.
The Language Level symbol shows a user's proficiency in the languages they're interested in. Setting your Language Level helps other users provide you with answers that aren't too complex or too simple.
Has difficulty understanding even short answers in this language.
Can ask simple questions and can understand simple answers.
Can ask all types of general questions and can understand longer answers.
Can understand long, complex answers.
This answer was given within 60 minutes of the question being posted.
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