Blocking (also known as Interference) is a tactical theme where you physically place a piece on a square that interrupts the line of communication between an opponent's pieces.
When an enemy piece depends on long-range protection from a Queen, Rook, or Bishop, you can "jam" the connection by stepping into the middle!
This move is often a sacrifice. You might offer a Pawn or a Knight just to block a defender's view for a single turn. If that one turn allows you to capture a much more valuable piece elsewhere, the sacrifice is a brilliant success.
In the second task, you will see how a Knight can jump into the middle of a long diagonal. By landing on d5, the Knight creates a "wall" that prevents the Queen on g8 from seeing her teammate on a2. This forces the opponent to choose between losing the defender or the piece being defended.
Think of it as "cluttering" the board. The more pieces your opponent has grouped together, the more likely they are to get in each other's way. Successful blocking turns an opponent's own pieces into obstacles that hinder their mobility and defense.