Patents can threaten the long-term viability of an open-source product. For example Microsoft sued Linux-using companies for patent-infringement. The apache license has a section on grant of patent license. This does not solve all patent issues, but is more than other licenses do.
The license basically allows anyone to do whatever they want with the code as long as the original copyright and license notice is included along with the copy of the code. The code can be used for commercially, privately, it can be modified and it can be distributed.
The MIT/Expat license doesn't protect against open-source code being taken (without payment) and used in proprietary software. This is harmful to user freedom because it lets future development be taken out of the public domain and instead moved into non-free programs.
In addition, it doesn't protect against software patents being used to attack user freedom. Unlike the Apache 2.0 and GPLv3 licenses, the MIT/Expat was written before software patents became a problem and doesn't include a patent release.
The license basically allows anyone to do whatever they want with the code as long as the original copyright and license notice is included along with the copy of the code. The code can be used for commercially, it can be modified and it can be distributed.