This is a discussion of a trap in the Ruy Lopez, Steinitz Defense. This is an important trap to know, because it shows what happens if Black tries to hold the center in the Steinitz.
When Tarrasch beat Marco in 1892, it signaled the beginning of the end for the Old Steinitz Defense. Today the Neo-Steinitz, 3...a6 4. Ba4 d6, is much more popular because it avoids the trap in this video.