It would really be best to work and think in absolute humidity, or the grams of water per cubic meter of air. You want to ventilate the house when the outside air has less absolute humidity than interior air.
However, nobody ever works in absolute humidity. All anyone ever talks about is this rather inconvenient unit called "relative humidity".
See, at any particular temperature, air has a limited capacity to hold water. For instance, a 10°C (50°F) temperature, air can hold 9 grams of water per cubic meter (assuming sea level, good grief). Likewise, 20°C (68°F) air can hold 18 grams of water per cubic meter (note: double, and we've only gone up 10 degrees). Any more will condense.
So if they give you relative humidity (which will be a percentage), you need to find out the absolute capacity of air at that temperature and now you will have grams per cubic meter. You get that from a place such as the chart that SteveSh posted in that answer above, except that chart is truncated off so it is useless above 25°C (77°F).
Then you multiply the air's capacity at that temperature by the percentage, and that gives you the "grams per cubic meter".
E.G. 50% relative humidity at 20°C means 18g x 50%. = 9 grams/m3.
33% relative humidity at 10°C means 9g x 33% = 3g.
Isn't that just perfectly awkward?
When outside air comes into your house, it will be the exact same grams per cubic meter that it was outside. However, as you warm it, the relative humidity will lower, and as it passively cools, the relative humidity will raise. If you mechanically cool it (air conditioning), the A/C evaporator is quite cold (maybe 2-5°C) and will condense a lot of the water out of the air.