Here is an informative breakdown of what this feature entails: 1. Defining the "Second Level"
Defensive tackles (often called "space eaters") take on double teams. Their job isn't necessarily to make the tackle, but to keep offensive linemen busy so they cannot "climb" to the linebackers.
When coaches talk about a defense's ability to "reach the second level," they are usually describing the chess match between offensive blockers and the defensive front.