Movement

Movement is the process which plays a role in deriving S-structure from D-structure and LF from S-structure by the reordering of constituents. We say that an element at S-structure has been moved if there is a certain relationship between the element and an empty position elsewhere in the structure.

Example
in the question Who did he see, we interpret who as the direct object of see. This is explained if who is generated in the direct object position at D-structure (to the right of see) and has been moved to the first position in S-structure. This is a case of overt movement because the effect of movement is visible at PF. When movement is involved in the derivation of LF, we speak of hidden or covert movement: the effect is invisible at the level of PF. Thus in a multiple question like Who saw what, the second wh-phrase what is covertly moved to sentence initial position in the derivation of LF (see wh-in-situ).

Links
Utrecht Lexicon of Linguistics