Understanding 1N5400 series datasheet
VRPM - Max repetitive peak voltage. Can be applied repeatedly. This is often taken as the usual value of the maximum reverse voltage you'd expect the diode to be able to block, but as it's in the maximums section, it would be prudent to derate somewhat.

I F(AV) - The average forward current. This governs the heating of the diode. That's why they specify a lead length and a temperature. 

IFSM - The short term surge. This is the sort of current you get into an uncharged capacitor when first switched on. It tends to be limited by the transformer impedance. It's very much larger than the average current, this is one robust diode.

VF - Forward voltage. We normally say 0.7v for silicon diodes, but this is specified for 3A so includes the effects of residual resistance.

IR - Reverse current, usually measured at maximum reverse voltage and maximum temperature, where leakage is worst. For some diodes, this can also be a significant source of diode heating.