The phrase "high density" refers to a situation or object that has a lot of things or people packed closely together in a small area. It means that there is a large number of things or people in a limited space. Full definition
These apparently isolated disturbances «can cause a phase transition to a state with very high density of vehicles which stays for hours on the road,» he adds. (sciencemag.org)
The pair members are associated with high density regions of galaxies. (sciencedaily.com)
This is not what is required to combat contemporary terrorists who typically operate underground in high density areas in the country they plan to attack. (blog.politics.ox.ac.uk)