Breakpoint resolution for copy number variation analysis
It means that it is hard to tell the precise location on the chromosome where the structural variation event starts and finishes using only read-depth information. 

Read depth is measured by counting the number of mapped reads that overlap targeted regions aka bins. These bins are typically many basepairs wide and are used to normalize coverage with respect to various parameters (e.g. GC content). Smaller bins can give better breakpoint resolution but have noisier statistics. Large bins have better statistics but can't be used to precisely call the SV boundaries.

The CNVnator paper discusses the topic
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3106330/