overlap in a strict sense means 2 or more things lap over each other partially. Or to put it differently they cover the same area partially. just google for images of overlapping to get a graphical depiction of this.
So 2 common uses of overlap in a metaphorical sense are
1) calendar entries. Mainly because how they overlap in the calendar, so you hear people say I have overlapping appointments or meetings etc.. Note: overlapping in this context normally means that meeting/appointment a doesn't finish until after meeting b has started. Normally in a manageable way i.e. just means you'll miss first 5 mins or so. If its longer or means you'll have to miss one then you normally call it a conflict.
2) Courses/areas of study, here the it is normal to talk about area/field of study, as such again it is typical to talk about overlapping fields of study. An example here is that biology and medicine are overlapping fields of study. i.e. they both cover a lot of the same stuff, this also typically means that they are complimentary. i.e. research in one can benefit the other.