What does phasing mean?
This refers to haplotype phase (aka gametic phase). This essentially means knowing which allele belongs to which copy of the chromosome, or alternatively, which alleles appear together on the same chromosome.

In short-read sequencing, for example, it is difficult to resolve the haplotype of two heterozygous SNPs if they have not been covered by the same read. If you observe A/a and B/b, you do not know whether you have AB + ab or aB + Ab. So you would say you do not know the phasing.

See the Wikipedia links for more information.