In this material world, the consequences of our actions and words, determine what kind of life of abundance and happiness, or lack of abundance (i.e. poverty) and suffering we live. All activity and speech is preceded by a thought, no matter how focused, unfocused, intentional or unintentional that thought is.
Take a moment to ponder these last two sentences. If you believe them, wouldn't you be much more careful about what you think in every breathing moment. If you don't believe them, I would like to hear from you as to what you think precedes action in humans.
The "foolishness" Yogananda refers to in the above quote is caused by going through the day allowing semiconscious thoughts to float though our minds. Yoga teaches us that although many of these unfocused thoughts do not lead us to immediate action, every one of them is "energy" and all "thought energy" leaves its signature in our chakras as a "vritti". It is these lapses in controlling our thoughts, and lack of concentration on our true essence, as higher spiritual beings that eventually contribute to negative desires and actions, causing negative karma and suffering.
Yogananda goes on to acknowledge the difficulty in keeping our thoughts on worthwhile activities. He describes those worthwhile activities to always include having our "...mind to be engrossed in Him (the Infinite Consciousness), in spite of any troublesome circumstances..." The difficulty, for a vast majority of human beings alive today, of keeping the Infinite God Consciousness in the forefront of our minds, is our childhood upbringing. As little children we tend to emulate our parents, our teachers, and those in our circle of friends. If you were taught by these early influential figures to keep the Infinite Consciousness in the forefront of you mind in all your daily activities, then you are indeed blessed by good karma to have been born into such a spiritual home. I have personally witnessed such children and homes in many of the Ananda Sangha Communities. These communities were founded by Swami Kriyananda, one of the few remaining living direct disciples of Paramhansa Yogananda.
Another of my favorite quotes from Paramhansa Yogananda is: "A saint is just a sinner who never gave up..." By constant vigilance over our thoughts and never giving up, we all can join the great self-realized saints an sages of all religions.