What does it mean to have low ovarian reserve and how does it affect my trying to conceive?

Ovarian reserve. Ovarian reserve is medical-speak for 'how young or how old do your ovaries feel, ' regardless of your actual age. 'low' or 'diminished' refers to ovaries that are getting tired, where fertility may be reduced. There are different ways to test ovarian reserve that don't always agree; also testing rarely guarantees you 'will' or 'won't' get pregnant.
Fewer eggs. Lower reserve means that you have less follicles in the ovary to potentially produce eggs. The ovaries seem to have a finite number of oocytes and thus less eggs to be available for fertilization, thus making it harder to get pregnant as there may not be an egg present when the sperm arrives.