What do curly braces mean in Verilog?

The curly braces mean concatenation, from most significant bit (MSB) on the left down to the least significant bit (LSB) on the right. You are creating a 32-bit bus (result) whose 16 most significant bits consist of 16 copies of bit 15 (the MSB) of the a bus, and whose 16 least significant bits consist of just the a bus (this particular construction is known as sign extension, which is needed e.g. to right-shift a negative number in two’s complement form and keep it negative rather than introduce zeros into the MSBits).

For what it’s worth, the nested curly braces around a[15:0] are superfluous.

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