The solar panel is given a very good protection against weather during the manufacturing process. But the lead out cables can become the Achilles heels of the panel. (For those who do not know, Achilles was a character in Greek mythology. When he was born his mother was told that he would die young. She somehow took him to the river of life and dipped him head down. But the heel she was holding him by was not wetted. When came of age he fought victoriously because nothing would harm him. But then a poisoned arrow hit that very vulnerable heel, and he died as predicted).
MC 1 and MC3 connector
Previously, the lead out wires of the solar panels would be screw tied to other panels or control equipment, in a simple junction box and would soon succumb to weather effects. National Electric Code (NEC) forbids people other than trained technicians to handle naked equipment at more than 50 volt. Solar panels in series invariably exceed this.
To overcome these two problems different designs were put on the market starting in about year 2000. Essentially, these were rubber shielded jacks and sockets which would push mate. Now those could be used by ordinary people. But they had no locking. These were what we call MC1 and MC3 connectors.
MC2 and MC4 connector: positive locking and ingress protection
NEC code was revised in 2008 to additionally require positive locking and ingress protection. MC2 and MC4 are again push fit types but have a positive locking mechanism and proper ingress protection as per IEC 29.
Normally a tool will be required to disconnect, but in some designs tabs can be pressed to unlock and pull. They will normally take AWG 10 or AWG 12 wires to crimp or solder. Current rating specifications by one manufacturer is about 30A, with contact resistance less than 0.5 Ω. Voltage is normally rated to 600 V in US and 1000 V outside US, and ingress protection to IP 67.
The letters ‘MC’ stand for Multicontact, and do not imply multiple contacts in the connector. Multicontact was the name of one of the original manufacturers which caught on. The digits 1 to 4 stand for the contact pin cross section in mm2.