CoachSIMD (single instruction, multiple data) is the execution of one operation on several data elements at once. A vector instruction is such an operation on a vector register holding several elements side by side: on x86-64, 16-byte xmm registers (SSE2, part of every x86-64 processor), 32-byte ymm registers (AVX2) and, on processors that have it, 64-byte zmm registers (AVX-512).
| stands for the SOH delimiter (byte 1). The comparison sets a byte to all ones where the message equals the delimiter; the mask instruction collects one bit per byte; counting trailing zeros gives each position in turn.