Ghosting means to fire tiles into spaces not occupied by enemy monsters or heroes.
Ghost tiles charge up your mana much quicker than tiles that strike enemies.
It’s also a way of clearing unwanted colours without charging enemy mana, which is increased with every hit.
In the context @Wharflord was describing, he/she was destroying most of the monsters in a wave, but leaving one alive. They would then fire tiles through the gaps in order to charge Belith’s mana and use her healing power. This meant she would start every wave with a full health bar. This is a smart technique and is useful for rare quests, like Sand Empire in particular.
Any other questions about jargon, try the Fictionary: