When a reaction rule is fired, or in other occasions where rules are used, you have the options to check a number of conditions and abort any actions if the conditions aren’t met. Conditions can be pretty specific, such as Content is promoted to front page, Data value is empty or the comparison between two text strings – but the most used condition is called Data comparison. This one is used to compare two variables available to Rules, or to compare a data variable to a manually entered value – for example to check if a field has a certain value or if the node author is the same as the acting user.
Conditions can be logically grouped to create and and or groups. These groups may in turn contain other logical groups.
There are two conditions that are particularly important when working with fields: Content is of type and Entity has field. These conditions allows Rules to know that an entity has a particular field, and subsequently makes the field data available in configuration. The condition Entity is of type has a similar role to provide entity-specific data, but is used much less frequently. (You can also use the Data comparison condition to check the entity bundle.)