When a cell divides by meiosis, it ends up with only half the amount of DNA it had before. That is a haploid cell. When two haploid cells join together to make one new cell with the regular amount of DNA, that's a diploid cell. All living creatures that use meiosis have to spend some time as haploid cells and then some time as diploid cells. This is called alternation of generations, because you alternate being a haploid creature and being a diploid creature.
Some creatures spend most of their time as haploid cells, and only become diploid for a short time before quickly going back to being haploid again. For example, moss is a plant that is usually haploid and only diploid for a short time.
Other creatures spend most of their time as diploid cells, and only become haploid for a short time. They quickly go back to being diploid. Your cells are all diploid, except for sperm and egg cells. They are haploid, but you only spent a very short time as a haploid sperm cell and a haploid egg cell before they combined to make you a diploid baby. All pine trees and flowering plants also spend most of their time diploid, and only a short time haploid.