species and taxonomy

it mentions is a textbook that 'when a species reproduces sexually, any of the genes of the individuals can be combined with any other' can someone explain what this means please

Why is this thread entitled 'species and taxonomy'. Is this relevant to the question you've asked above?

I was wondering that too, I assume that's just the topic of the section of the book OP was reading from.

It sounds like it's referring to recombination and independent assortment.

In gametogenesis, recombination between maternal and paternal chromosomes can put some maternal alleles on the paternal chromosome and vice versa so the alleles (of the parents of the organism forming the gametes) can be combined with any other in the gametes.
Independent assortment can put maternal and paternal chromsomes into gametes regardless of which source (mother or father) they came from. This means the alleles on the maternal chromosomes have just as much chance to go into one gamete as into the other during meiosis, and the same for the paternal chromosomes.

Alternatively, since gametes of the same organism will be different because of the above processes, and it's essentially up to chance which two gametes meet (excluding formation defects), the combination of the gametes that meet is random - so any existing genetic combination of one gamete can be combined with any other existing genetic combination. This sounds like an awkward explanation to me, so let me know if what I'm saying isn't clear.