The measurement model is a part of the model that represents relationship between the latent variables and their measures (manifest variables). It is generally called the outer models in PLS-SEM.
The basis for determining these relationships is measurement theory.
In short, measurement model assessment aims at ensuring construct validity & reliability.
Validation of the measurement models is a requirement for assessing the structural model.
Random error (e.g., caused by the order of items in the questionnaire or respondent fatigue) (Heeler & Ray, 1972)
Systematic error, such as method variance (e.g., variance attributable to the measurement method rather than the construct of interest) (Bagozzi et al., 1991)
Xm : Value as measured
Xt : True value
ε : Error terms
εr : Random error
εs : Systematic error
Thus, the main objective of measurement model assessment is to reduce the effect of measurement errors in a data-set.