In a sentence, the part that provides information about the subject is called the predicate. In Japanese, this is known as 述語(じゅつご). い-Adjectives can function as the main predicate, standing alone when the subject is implied or concluding a full sentence.

• 安(やす)い。
  It's cheap.
• この店(みせ)は安(やす)い。
  This store is cheap.

When an adjective is used by itself, it conveys a complete thought much like English expressions such as 'It's cold' or 'It's delicious'. The listener must infer what 'it' refers to based on the surrounding situation or conversation.

The particles は and が can both mark the item being described by the predicate. The most natural choice between them is determined by the context. As a general rule, が is preferred when you want to single out something and emphasize its quality in contrast to others of the same type.

• この荷物(にもつ)は重(おも)い。
  This luggage is heavy. (A general description)
• この荷物(にもつ)が重(おも)い。
  This luggage is heavy. (In contrast to other, lighter luggage)

In this context, が identifies this specific piece of luggage as the one possessing the quality of being heavy, distinguishing it from the rest. The particle は would be less fitting in a situation where such a comparison is implied, as it's typically used for more general, established facts.