The warmer the air, the more moisture it can contain. At freezing point, a cubic meter of air can't contain more than 5 grams water. But at 15 C, which is the average temperature on earth, it is up to 13 grams per cubic meter.
In the first case, the absolute humidity is 5 gr/m3. In the second case, 13 gr/m3. But in both cases, the relative humidity is 100 percent because the air is then entirely saturated.
During the summer, the absolute humidity is higher but do we feel it? Not very much. The human body is sensitive to the relative humidity because when it is 100 percent, no evaporation occurs and if e.g. we sweat, it doesn't cool down the skin since it doesn't evaporate.
However, the absolute humidity e.g. tells how static electricity can stay in the molecules of air and since water leads electricity, less moisture means more static electricity during the winter.
Recently medicine has also discovered that the reason the flu and the common cold is more frequent in the winter is due to the low absolute humidity, which increase the survival of the virus in the air.
Last but not least; when the air is cold outside and it enters our heated homes, it gets very dry. If we take e.g. the example above: freezing (0C) air at 50 percent relative humidity would mean 2.5 gr/m3 absolute humidity. When that air enters a home that is heated to 15 C, 2.5 gr/m3 represents only 20 percent of saturation point of 13 gr/m3. 20 percent relative humidity is felt as dry air!