UDIMM Motherboard

Hi, I have question about ram for ASrock B360M pro4 motherboard. On ASrock site for B360M pro4 motherboard in technical specification section says that "Supports DDR4 2666 / 2400 / 2133 non-ECC, un-buffered memory". Ok I know that ECC is server side RAM but I am not sure about "un-buffered...

The other day I tried to replace my cpu on my thinkserver ts140 It's a i3-4130@ 3.4ghz LG 550 socket ( I believe) anyways I had the wrong socket type i5 cpu I tried to install it didn't work so I took it out and placed the old one back in and now my pc doesn't power on. The fans spin but...