I think that I can provide some information on the first two items, but the last one is well above my pay grade and would only be speculation based on impoverished internet research.
First of all, I am sorry that you had to experience a bone marrow biopsy. Although I have had over 20 of them, I know that they are not pleasant and can be a little nerve wracking, especially while awaiting results on what it all means.
The first item listed was marrow cellularity of 40%, which you stated was mildly hypo-cellular. Basically, this is a reading of how dense your bone marrow is. Normal cellularity is a range, but the rule of thumb is that you take 100 and subtract your age for the target percentage. For example, if you are 40 years old, you would take 100 - 40, and expect about 60% cellularity. Anything above might be HYPER-cellular and anything below might be HYPO-cellular. Really low cellularity leads to reduced blood cell production and can result in aplastic anemia of varying grades.
The decreased iron stores means that you may have some degree of iron deficiency, but there isn't a reason directly associated with it from this observation.
The second item was mixed population of b and t cells. These are types of white blood cells and is merely an observation that you have both, which is normal. Since there wasn't really a qualifier, it did not appear that you had an abnormal imbalance between the two, and the fact that they were maturing indicates that their development appears normal. (again, only based on the statement provided).
The third observation is where this gets tricky, and it is really hard to say as my experience is with MDS-type marrow results. A quick search suggests that hematogones are early stage B cells that look like lymphoblasts and they represent 2.3% of the (presumably) b cell population. I have no idea what this means from a diagnostic perspective and strongly recommend asking your doctor directly what he or she is thinking of as possibilities and what steps, if any, are required going forward.
MDS RCMD w/grade 2-3 fibrosis. Allo-MUD Feb 26, 2014. Relapsed August 2014. Free and clear of MDS since November 2014 after treatment with Vidaza and Rituxan. Experiencing autoimmune attack on CNS thought to be GVHD, some gut, skin and ocular cGVHD. Neuropathy over 80% of body.