According to John Oliver's video on prisoner reentry, what is mass incarceration? Why does it happen? What are the causes of it? Why is this a problem? Texas has one the largest prison systems in the United States. What can we do to fix this issue?

Mass incarceration is the imprisonment of an historically large proportion of Americans. Today, 5% of the population is in jail, which is five times the proportion of Americans who were in prison in the 1970s. Among the causes of the mass incarceration are federal laws regarding criminal justice, including mandatory minimum sentencing laws for certain crimes. These laws were passed in response to the growth of crime in the late 1960s and early 1970s, and the political response that called for "getting tough on crime." Both major political parties, Democrats and Republicans, have passed these laws, particularly before elections, to gain favor with the public. 
The problem with this system, as John Oliver's video points out, is in part the very high rate of recidivism. This means that each year an inmate serves time in prison increases the chance he/she will commit another crime. According to the video, the national recidivism rate is 50%. That means that 50% of inmates will return to prison after being released. Released inmates are given very little money when they are released, and they also have to indicate that have been convicted of a crime on job applications, reducing the chance that they can get a job and abide by the laws governing their probation.
To fix prison systems like the one in Texas, alternatives to prison should be considered. For example, some people would do better in school programs, drug treatment centers, or mental health treatment programs. In addition, the public defender system is very poor in some states, meaning that accused people do not get a fair trial. If public defenders were better paid, the system would improve.