Pollination is the act of moving pollen, which contains the sperm of a plant, from the anther (male organ) of the flower to the pistil (female organ) of the flower so that the plant's egg can be fertilized to produce a seed. If the pollen is from the same plant -sometimes it's even from the very same flower - the process is called self-pollination or autogamy. If the pollen and egg come from different plants, then the process is called cross-pollination or allogamy. Cross-pollination helps to maintain genetic diversity, which gives the species more resiliency under changing environmental conditions.
In order for cross-pollination to occur, two plants of the same species must be in bloom simultaneously, one with pollen available for distribution, and the other with ripened eggs. Then the pollen from the male parent must be moved so that it reaches the stigma of the female parent. How this happens depends on what pollination schema the plant is adapted for.
Pollen can be moved by wind or water, or transported from one plant to another by insects, birds, or bats. Every plant species has adaptations for a specific mode of pollen transfer. If you would like to learn more about the different pollination strategies, have a look at this link.