This chart shows a distribution of the variant types present in the selected VCF file. SNPs are by far the most prevalent type of genetic mutation, so it is expected that these dominate the distribution.
For whole genome sequencing data, SNPs dominate, but there is typically an appreciable number of insertion (Ins) and deletion (Del) mutations, as well as Other variant types. These other variants include anything that doesn't fall into the previous categories. For example, consider the case where the reference sequence is ACTG, and sometimes the A is mutated into a C, but whenever it is, the T is always deleted. Rather than these two mutations appearing separately in the VCF file, since they are only ever observed together, the reference will be ACTG, and the alternate will be CCG (the A mutated to C, and the T deleted). This mutation is a combination of a SNP and a deletion, and is listed as a complex variant type and will appear in the Other category.
It is much less likely to find structural variants, when looking at whole exome data. Large events are often not detected since the breakpoints fall outside of the exome sequencing regions. Also, insertions in, or deletions of large coding regions are likely to be more deleterious than in the non-coding regions captured in whole genome sequencing, and so there are less present in these regions. For these reasons, SNPs are expected to dominate the variant types distribution even more in exome sequencing.
This chart is primarily used to check that all expected variants are present. If you are expecting to see a complete VCF including insertions and deleted, but the file has been filtered to leave only SNPs, this chart will quickly help identify this.