Durability blocks are reserved vanilla blocks with extra functionality. As the name implies, they have a durability, or health value, associated with them. Different blocks have different durability values, depending on how strong they are. Your base should be built out of durability blocks, or other players could simply mine their way in.

Upon being placed, a durability block will start at 0 health. Over the course of about a minute, it will steadily gain HP in a linear fashion until it reaches the max, at which point it will stop gaining HP. This principle is called 'settling in'. If the block is interrupted while it is settling in, like by taking damage, it will stop settling in and will no longer continue to gain durability. In this case, the player has no choice but to either destroy the block completely, or Repair it.

Durability blocks will never drop themselves when destroyed, even if being 'fast-mined' within a Lockbox region. This includes special blocks like Custom Blocks and beds. Certain durability blocks are resistant to fire damage or being mined.