I think I can help on this one.
First sentence: "Most of the modern TBs & SDBs are 48 c." This means that modern tall bearded irises ("TBs") and standard dwarf irises (SDBs) have 48 chromosomes ("48 c"). This is double the normal count for irises, hence they are "tetraploid".
Second sentence: "MTBs are diploid & more tets are coming. the hybridizers usually indicate." This means that miniature tall bearded irises (MTBs) are diploid (24 chromosomes) but hybridizers are creating more of them that are tetraploid (tets) and the hybridizer would include this information in the description.
A short explanatory dictionary to summarize:
Diploid iris have 24 chromosomes
Tetraploid (tet) iris have double that, 48 chromosomes
TB = Tall bearded iris
SDB = Standard dwarf bearded iris
MTB = Miniature tall bearded iris. These are tall irises that have little flowers.
All of this is important to know for hybridizers. For a cross to be successful the two parents need to have the same chromosome count, for example, both diploid or both tetraploid. Otherwise it is not important to know if you are just growing them to look pretty in the garden.
Well now that I have muddied the waters more....
