Before use pH meter must be calibrated. Each pH electrode used for measurements is slightly different and its characteristic changes with aging. Without proper pH meter calibration results will be usually off by at least several tenths of the unit. pH meter calibration procedure calls for use of two or three pH calibration buffers of exactly known pH (these can be bought as either solutions or in solid form) in which pH electrode is dipped and pH meter indications are corrected. Depending on the pH meter type it may either recognize buffer automatically and perform calibration procedure almost on its own (just asking for buffer change when needed) or you will have to calibrate it using knobs and changing buffers once each calibration step is completed. In both cases underlying principle is the same - gain and offset parameters are set assuming linear dependence between pH and electrode voltage.
pH meter calibration is more often referred to as pH electrode calibration, as parameters set are not device dependent, but electrode dependent. Correct procedure of the pH meter calibration is usually described in the pH meter manual. General outline of the procedure is presented in the pH electrode calibration section.