Bija means seed. Bija mantras mean seed mantras.
The good explanation of mantra is given in Sir John Woodroffe’s collection of essays, Shakti and Shakta, chapter 24.
“The supreme absolute (Parabrahman) exists in the human being (jivatma) as Shabda Brahman, the absolute as sound. Mantras are not prayers and the relationship of the letters of the Sanskrit alphabet, whether consonants or vowels, he says, point to the appearance of devata (divinity) in different forms. An uttered mantra is the manifestation of a more subtle sound while mantras themselves are forms of Kundalini. Mantras may be male, female or neutral. Female mantras are called Vidyas.”
For realization of Lord Ganapati, knowledge, wisdom, protection, health, wealth, elimination of all obstacles.
For realization for Goddess Durga, power, strength, protection, health, wealth, victory, wisdom, knowledge, elimination of major problems in life.
For realization for Goddess Mahalaxmi, wealth, material gains, success.
For realization for Goddess Kali, health, strength, protection, elimination of enemies.
For realization of Goddess Saraswati, knowledge, wisdom.