Some reactions hold their own conditions and some reactions take their own actions and most reactions do both.
For example the reaction restart world from the Game manager building block is a reaction that takes only action and has no condition. Without a condition the reaction can only be called with the help of another reaction that has a condition.
A reaction that has only a condition is the reaction collision form the player building block. This reaction does nothing when it is called but can be used to call another reaction that does take action.
There are also reactions that do have their own condition and also take their own action.
When you select a reaction you will have to choose it's followup. You can choose set property or set reaction as a followup.
If you choose set property you can change the value of any property from any building block as the followup of the reaction.
For example you can select the reaction collision from the player building block. As a followup you set the variable life of the player to 0. As a result the player will instantly die whenever he hits anything in the game.
If you choose to set reaction as a followup you will choose another reaction which does take action and will be triggered by your first reaction.
For example you can select the reaction collision from the player building block. As a followup you call the reaction die from the player building block. Again as a result the player will instantly die whenever he hits anything in the game.
To find out more about the reactions of all the building blocks you can refer to the building block reference list.