Answer:
When pollen from the pollen part of the flower (the anther) gets its way into the germination part (the stigma) of the SAME plant.
It's kinda weird because many plants have both "male" and "female" seexy bits on the same flower.
Step-by-step explanation:
Self-pollination is when pollen from the same plant arrives at the stigma of a flower (in flowering plants) or at the ovule (in gymnosperm-style plants).
There are two types of self-pollination: in autogamy, pollen is transferred to the stigma of the same flower; in geitonogamy, pollen is transferred from the pollen part (anther) of one flower to the germination part (stigma/ovule) of another flower on the same flowering plant.