Mosfet stands for Metal-Oxide Semi-conductor Field Effect Transistor. They are a part of the Voltage Regulator Module (VRM). The VRM controls the amount of energy your CPU takes in. The VRM also consists of Chokes and Capacitors which are driven by a "driver integrated circuit" (Driver IC - which can be a type of Digital PWM.) Capacitors store and help regulate the flow of energy, Chokes make sure that the electrical current stays within a certain range, while Mosfets do the physical work of allowing the CPU enough voltage. That said, mosfets are, by far, the hottest component of the VRM, and if a heatsink is present it is always attached to the mosfets.
A 4+1 phase means that four phases are dedicated to the CPU, while one is for the memory. As a general rule, more phases are better - it means that the VRM can handle more voltage, and is more capable of providing stable energy to the CPU. However, this is not always the case. Biostar's TA990FXE motherboard has a 4+1 VRM, but they used some quality components that can handle a 300 watt load. Meanwhile, there are some 8+2 VRMs that are "doubled" 4+1. That's, uh, a different story.
MSI's AMD entry-to-mainstream level motherboards were... ill equipped. Not only did they usually not have too many phases for their VRMs, but they had no overheating or failsafe features. So a lot of people were running hard overclocks on hardware not meant for it, which led to catastrophic failure. The 970 Gaming "fixed" a lot of issues, but some people still managed to bork the board when trying to run a sustained 5.0ghz. 4.4ghz should be perfectly fine, though.