Chromatography is a dynamic separation process based on differences in the rates of migration of different compounds through a column containing an immobilised stationary phase. Migration is induced by a  continuous flow of a mobile phase through the column. Sample components are only transported through the column during the time that they are present in the mobile phase. The mobile phase flows through the separation column unretained, while the sample components are retained by the stationary phase.

In order to separate two sample components, there must be a difference in their retention behavior. In general, chromatography is based on the selective distribution of the different components between two phases. One of the phases, the (stagnant) stationary phase, is held immobilised inside the column while the other phase, the mobile phase, is flowing through the stationary phase along the column.

In LC the stationary phase is present in a column as small particles with an active surface. Compounds that exhibit a higher affinity for the stationary phase will travel more slowly than compounds exhibiting a lower affinity. 

The greater the affinity for the stationary phase, the higher the retention. This results in differences in retention between different sample components travelling at different speeds through the column and eluting from the column at different times. The column should be sufficiently long to obtain the desired degree of separation. See the animation below: