A block (pulley) is a wheel on a shaft mounted in a housing used for lifting objects or changing the direction of effort.
Block and tackle refers to using multiple pulleys, one fixed and the other moving to create a mechanical advantage.
The weight is reduced by (number of passes -1), but the will need to be pulled (number of passes-1) times the distance. For instance, a tackle with 5 passes of the line can lift 200 pounds of load with only 50 pounds of effort, but, it will require 20 feet of line to pass through the tackle to lift the load 5 feet.
- Image by Welkinridge - Own work by uploader based on a diagram in "Notes on cargo work" by Kemp and Young. 1977, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4227455