This accounts for the block header, var_int encoding of the transaction count and a minimally viable coinbase transaction. It adds an additional safety margin, because even with a thorough understanding of block serialization, it's easy to make a costly mistake when trying to squeeze every last byte. Setting a lower value is prevented at startup.
Declared in <policy/policy.h>
inline constexpr unsigned int MINIMUM_BLOCK_RESERVED_WEIGHT = {2000};