In HPLC the mobile phase is a liquid. It is the phase with which your sample/substance is brought into the HPLC system and moved through it. Different mobile phases have a different strenghts of elution and this causes variable retention times and selectivities. In HPLC we divide into mobile phases for reversed phase or normal phase applications. Typical solvents for reversed phase use are mixtures of methanol and water or acetonitril and water. Normal phase eluents can be hexane or heptane for example.