In sexual reproduction a male gamete and a female gamete from two parents fuse at fertilisation. Each gamete carries half of the genes needed, so the offspring inherit a mixture of genes from both parents. This means the offspring are not identical to either parent or to each other.
The main advantage is variation in the population. If the environment changes, or a new disease spreads, some individuals may have features that help them survive it. They live to reproduce and pass on those features, so the species is less likely to become extinct. Variation also gives natural selection something to work on over many generations, and in farming it lets breeders choose the best plants or animals to breed from.
There are disadvantages. Two parents are needed, so an organism must find a mate. This takes time and energy, and it is harder when individuals are few or live far apart. The reproductive cycle is usually slower than in asexual reproduction, so numbers rise more slowly.