What are the prayers on a Buddhist prayer flag about?

Traditionally prayer flags promote peace, compassion, strength, and wisdom.

The prayers or sutras which are written onto the flags do not actually carry to the Gods, as many misappropriate however Tibetans believe they are carried by the wind towards the world and whoever needs the prayers the most.

Each prayer flag contains a set of 5 colors (Blue, White, Green, Red, Yellow) which symbolically represent the 5 elements (Sky, Wind, Fire, Water, Earth). Even with the different types of flags such as

Even with the different types of flags such as lungta, darchoks, and wish-fulfilling flags, this system of color coordination stays consistent and keeps the harmony of the flags

How to spread Peace, Compassion, Strength, and Wisdom

Prayer Flags are auspicious flags that contain images of deities, mantras, and prayers, which are traditionally believed to bring happiness, peace, prosperity, good fortune, health and victory over the obstacles. Prayer Flags are used to promote peace, compassion, strength, and wisdom. Prayers inscribed on prayer flags protect from destructive evils and obstacles toward the flag planter and all sentient being. Tibetans believe that the prayers and mantras will be blown upwards as an offering to their deities and will bring fortune, happiness and good health to all who hang them.

Lung-ta, Darchok, Healing

Lung-Ta is a Tibetan word meaning wind (Lung) horse (Ta). Lungta is a mythical animal who carries all intentions and prayers to the universe. As the wind blows the prayers fly out to the universe and are sent out to be fulfilled. Lungtas are square or rectangular shaped and are connected along their top edges by a long string, and are commonly hung on a diagonal line from high to low places, such as trees, stupas, or mountains.

Darchok translates to Flagstaff and originates in the Amdo region of Tibet. It is most commonly placed on the ground, on top of mountains or on rooftops. The main significant difference between the vertical and horizontal flags is that if there are no trees or structures to hang the vertical flags, it is easier to plant the vertical flag instead. 

Medicine Buddha is revered as the most powerful blessing for healing, dispelling sickness and for awakening inner healing. His body is blue, the left hand is in the meditation mudra, and holds a begging bowl full of healing medicine. His other hand is also outstretched offering help and healing. Hanging this prayer flag is very beneficial as placing this in your home or garden will allow the healing mantras on the flags to be sent out and spread peace, this is particularly beneficial as a gift item to someone who is sick. "Om Bhaisajya Bhaisajya Maha Bhaisajya Rajaya Samudgate Svaha" is the mantra of the Medicine Buddha. If one recites this mantra, one can eliminate the sickness of body and gain a long life.

5 Colors : 5 Elements

In Vajrayana Buddhism the outer cosmic world is divided into 5 basic energies, and in the physical world they manifest as space, air, fire, water and earth, and life on Earth is comprised of these 5 elements. It is believed that spiritually these 5 energies are also connected with the 5 Buddhas and the 5 pearls of wisdom. Prayer flags attempt to follow this very natural system as well by associating each of its 5 colors to each element. (Blue-Space) (White-Air) (Red-Fire) (Green-Water) (Yellow-Earth).