Vectorization means that the compiler detects that your independent instructions can be executed as one SIMD instruction. Usual example is that if you do something like
for (i = 0; i < N; i++) {
a[i] = a[i] + b[i];
}
It will be vectorized as (using vector notation)
for (i = 0; i < (N - N % VF); i += VF) {
a[i : i + VF] = a[i : i + VF] + b[i : i + VF];
}
Basically the compiler picks one operation that can be done on VF elements of the array at the same time and does this N/VF times instead of doing the single operation N times.
It increases performance, but puts more requirement on the architecture.