A Low Block
A low block is a block that sits on, or around, the edge of the penalty box
At some point in games, all teams will adopt this block, as it makes it really hard for teams to get into good goal scoring opportunities.
The central area around the goal is always heavily manned, and any balls played into this area are aggressively defended.
When a low block has is being adopted, a team will quite often use a 5-4-1, 4-5-1 or sometimes 5-5-0 structure.
As a philosophy of play it is often refereed to as 'parking the bus'.
When teams play this way, counter attacks are a big part of a teams attempt to get forward forward and win games.
Jose Mourinho has won champions league titles using this philosophy of play