codominace: you have red and white flowers and when you cross them, if you get a one that has red and white spots (means carries both of the parents’ characteristics together) this is codominance. I can give you the AB group (blood) as an example.
multiallelic traits: When many genes are possible for a trait. As an example I can again give the ABO blood system. A,B,O is possible. But note that the locus can have only 2 of them.
It took me a really long time to understand polialelism. As i was thinking it, all the alleles were present in one individual. This is wrong, i know it now. Poliallelism means more alleles on the level of population, each individual has 2 of them