Spot metering does exactly what it says - it meters at a single point only, usually the middle, not sure if some cameras let you change it. You would use this if the subject was very different from the surroundings and you want the subject to be right, and don't care much about the surrounding. So if you point at a bright subject, the rest of the scene can come out very dark.
Also note you can do exposure lock, so you can meter at one point and recompose. But I don't think you can lock in manual mode since as before you don't directly set the exposure.
_________________Canon DSLRs: 7D2, 7D1, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L, 100L, MP-E 65, EF-S 10-18, 15-85
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 50/1.4A, 150 macro, 120-300 f/2.8 OS
Compacts: Sony HX9V, Fuji X100.