In chromatography a liquid is pumped through a bed of particles. The liquid is called the mobile phase and the particles the stationary phase. A mixture of the molecules that shall be separated is introduced into the mobile phase.
In the animation below the blue molecules shall be separated from the red molecules and a mixture containing these two types of molecules is introduced into the mobile phase in front of the stationary phase. The mixture of the red and blue molecules is then transported by the mobile phase through the stationary phase.
The molecules in the mixture that adsorbs the most to the stationary phase, in this particular case the red molecules, is moving slowest through the particle bed. The red molecules become separated from the blue!