Syntax - [ - Begin List Literal
Characters: [
The beginners' page has a more thorough explanation of Jelly syntax and tokenization.
Functionality
Begin a comma-separated list literal. These are not required for a depth-1 literal, but can nest.
Details
[1,2] is equivalent to 1,2. You can nest these to create multidimensional lists (such as [1,0],[0,1] for the 2×2 identity matrix) or create singleton lists (such as [1]). Square brackets must be balanced. Note that [] doesn't work; instead, use “” or ⁸ (Chain Left Argument / Empty List) if in a niladic chain.