An in-text citation provides your reader with two pieces of information:
Smyth makes a similar argument (3-4).
This point has been argued previously (Smyth 3-4).
The article "Black Workers Matter" links racism and union representation (18).
The link between racism and union representation is important ("Black Workers Matter" 18).
Later, the protagonist of Jane Eyre proclaims, "I would always rather be happy than dignified" (413).
(Smith, Jones, and Brown 323)
(Bia et al. 161)
(K. Shepard 36)
(J. Shepard 212)
(Pushkin, ch. 5)
For more detailed information see MLA Handbook, 54-58, 116-128.