Basically what well-drained means is that the soil doesn't stay saturated for extended periods of time but rainfall and irrigation water drains away quickly enough that the plants don't "suffocate". When the pores in the soil are full of water the plant is deprived of oxygen (roots need oxygen), some plants will tolerate this for longer periods than others, and some become unhappy much more quickly. So you don't want to plant anything that needs well-drained soil where excess water takes a long time to drain away.
