Conditionals or as they are better know; If/Then statements.  If something happens then something else will happen.  Conditionals make things happen to your sprite while your script is running without interaction from you.

To start go to the Control section of the blocks.  We are going to start with the first one if <_> then.  You will see a hexagon shape in the center this is to put another block inside, called nesting.  A good example is to put your blue sensing block “touching” hexagon inside your if/then block.  This will read:  if <touching edge> then…have sprite do something like play a sound or move.