"Simple living"—as it applies to the philosophy and practice of Krishna consciousness—can mean many things, such as freedom from greed and extravagance, straightforwardness in social dealings, exclusive service to the Supreme Person, honest work in harmony with natural laws, and dependence on God's mercy.
The English poet William Wordsworth wrote,
Plain living and high thinking are no more:
The homely* beauty of the good old cause
Is gone; our peace, our fearful innocence,
And pure religion breathing household laws.
Srila Prabhupada may have had this poem in mind when he frequently used the term "simple living and high thinking" to describe the ideal way of life for people in general and devotees of Krishna specifically. "Simple living" is rooted in the idea that an intelligent person should spend as much time as possible cultivating spiritual awareness and not get carried away by material circumstances or desires.
Simple living requires minimizing bodily needs and accepting whatever comes by the arrangement of Providence. To this end, Srila Prabhupada always spoke in favor of local self-sustaining agrarian economies to meet life's needs without the hard labor needed to acquire extraneous amenities.
*In this context, the word "homely" means "unsophisticated and unpretentious," qualities even more rare now than in 1802, when Wordsworth wrote it.