Basically in TLC, you have a stationary phase such as : Glass/metal sheet covered in a thin layer of absorbent material e.g. Silica gel,

where different components in the sample can
adsorb to. Then you also have a mobile phase which would separate the components of a mixture along a stationary phase.
You place the sample on what is known as the 'origin' and wait for the different components to separate along the stationary phase.
The

value is equal to

(the solvent front is the furthest place where the mobile phase 'climbs' up to) (Note: the

value will always be smaller or may equal to 1)
The

value can be varied according to:
• Differing polarities of components in a mixture and hence differing solubilities
• Separation of the mixture by components adsorbing onto the stationary phase, and desorbing into the mobile phase where it will be swept along the stationary phase.
• Greater desorption implies a greater distance moved along the stationary phase.