what is double and single fertilisation?

Double fertilization is a characteristic feature of flowering plants. In the process of double fertilization out of the two sperm nucleus, one sperm nucleus fuses with the egg nucleus to form an embryo ( process is called syngamy) and another fuses with polar nuclei to form endosperm ( process is called triple fusion). Since two kinds of fusion—syngamy and triple fusion—take place, the process is known as double fertilisation

Single fertilization is the simple fusion of the male and the female gametes with each other to give rise to a single-celled zygote. The zygote then undergoes repeated divisions to give rise to a new individual.

Hope this information will clear your doubts about the topic.