So what does "Spun a Bearing" mean?

OK, ya got your pistons ramming up and down in your cylinders.  Pistons are connected to the crankshaft with connecting rods.  Where the rods connect to the crank, there are these round bearings that are filled with oil under pressure. 

Problem is, bearings have to be in there just right.  Under bad circumstances, they "spin" and rotate in their retainers causing the oil pressure to bleed off as the bearing wears unevenly. 

This causes a loss of oil pressure in the rest of the motor, which - you guessed it - will cause the bearing on the NEXT connecting rod to spin or otherwise begin to wear uneven, causing more loss of pressure...etc.