Bad planetary influence or bad planetary effect is caused by the bad karmas or deeds that you have done in your past life. Bad planetary influences are nothing but the weak points in one's life. These are the situations when you are trying hard to get out of it but because of one or the other unknown reason you are not able to make up. Like for an example you are going through the financial crisis and has good education but not able to get job. You are trying extra hard to get that but still, there is no positive result. This is all because of the bad deeds of your past life and malefic position of planets which is defined at the time of your birth on the basis of your past karmas.
Apart from this there exist certain doshas because of the bad planets in your horoscope. These are manglik dosha, kuja dosha, kaal sarpa dosha et al. All are rectified by their specific pujas, offering to God and charities. Say if you have Mars as a bad planet that it is better not to take important decisions on Tuesday.
Bad plan etary effect can be lessen by three modes.Mantra: In this the particular mode, mantra is recited to lessen the ill effects of a specific planet. Here the combination of the sound alter the bad planetary influence.
Tantra: Some of the bad planetary effect are so harsh that these cannot be subdued with the mantra. Here one needs to perform tantra as well.
Yantra: Yantra are nothing but the secretive drawings that are made in a very complex form. The yantra are worshiped for the long period of time because of bad health, distance or deficiency of time.
Apart from these you can wear gem stones and metals to lessen the effect.