In normal reproduction, the sex cells are haploid (n) meaning the offspring gets half the set of one parent and half the set of the other.
In plants you can have a full set of those sex cells passed on, they're called diploid (2n) gametes.
So say you have an ovary full of diploid egg cells (2n) and a regular haploid(n) sperm cell, the result will be offsprings with one full
set of chromosomes from one parent and half the set from the other.
It's called triploid (3n).
Triploids have usually larger flowers, good growth, number of flowers can be increased but they are usually sterile and cannot be used reliably for breeding.
So now if you have an egg cell that is diploid (2n) that encounters a sperm cell that is diploid (2n) you get a tetraploid (4n).
Tetraploids produce larger flowers with great shape that last longer, the plant can adapt to a wider range of growing conditions, the plant is overall stronger.
That's kind of a general answer but I am sure you get the idea.