The phrase "overcrowded prisons" means that there are too many prisoners in a jail or correctional facility, causing it to be very crowded and filled beyond its intended capacity. Full definition
There are fewer opportunities for rehabilitative work in overcrowded prisons due to a lack of supervision and inmates are confined to their cells for longer, causing greater tensions between prisoners and with prison staff. (politics.co.uk)
Throughout my career in Congress, I have worked to try to update our laws so that they better reflect our country's priorities - priorities that I believe should be focused on keeping our communities safe instead of overcrowding our prisons with low level offenders and harming the futures of many young Americans because of what often amount to minor offenses. (votehemp.com)
The most overcrowded prison in England and Wales, according to official figures, is Shrewsbury, which currently holds 316 men, despite being designed to hold just 177. (politics.co.uk)