Angie, where you are buying the house is a big part of the answer. Like any material, wood expands and contracts with changes in temperature and humidity. Wood planks experience more variations in width than they do in length, but both dimensions change. Normally, the gaps tighten during the summer and are necessary so the expansion has some place to go.
It is also possible that, if you are buying an old house, the planks were not dried sufficiently when they were installed. With todays kiln drying, this is much less a problem and flooring manufacturers have devised other methods to deal with expansion and contraction.
Another piece of the puzzle is are the planks tongue and grooved or are they on a subfloor. If either of these conditions exist, I would not be too concerned, but you need to know more before this really can be answered.