If I could play on Homer's Lyre
And wing with true poetic fire
To what great theme might I aspire?
Foliar feeding is simply feeding plants through their leaves, rather than through their roots. An analogy could be the intravenous feeding of glucose to humans. The nutrients reach the patient more rapidly.
Next question is: What do we feed?
Answer: Pretty much the same things you would feed through the roots although even more care should be taken to avoid feeding too strong a mixture of nutrient.
I like to use a compost tea that I make myself although one may certainly use a prepared soluble fertilizer such as Miracle Grow All Purpose or Peter's Blend All Purpose. In either case I recommend mixing at half the recommended concentration or less. Either concoction is strained to remove stray solids and then sprayed directly on the plant's leaves in a fine mist to the point of runoff. If the plants are in direct sunlight it is best to do this on a cloudy day or after the sun is low, not at high noon in full sun.
Recently (in the ten years prior to this writing) an interesting variation of this method has really taken hold, even finding its place in commercial agriculture. Manufacturers vie for this commercial market and several offer systems ranging from very expensive and elaborate to simpler, more economical, kits. I am referring to the foliar application of Aerated Compost Tea or A.C.T., as it is often abbreviated. The basic idea is to make compost tea and then aerate it for about twenty-four hours to encourage aerobic organisms to multiply. I accomplished this by putting the compost tea in a five gallon bucket and aerating it with an aquarium air compressor (purchased at a discount store for $6 U.S.) and a brick to disperse the bubbles. Proponents claim that the resulting brew will inoculate plants that it is applied to with beneficial organisms crowding out harmful ones. This, of course, it does in addition to foliar feeding. Naturally, arguments abound on both sides as to the efficacy of A.C.T. and I can't say that I myself have empirically proven the method. I will attest that it did no harm and I feel that I may be preventing some of the fungus diseases that are common in our humid climate.
Whether you choose to try aerating the foliar feed or not, I encourage you to experiment with this satisfying method of providing a "shot in the arm" to your plants. You just may win a blue ribbon!