A predicate is a word that describes the topic/subject of a sentence. The Japanese equivalent of this is a 述語（じゅつご）, which literally means ‘statement word’. As a predicate, い-Adjectives can appear by themselves (when the word they are describing does not need to be stated), or they may be used at the end of a sentence.

• 丸（まる）い。
  It’s round.
• 地球（ちきゅう）は丸（まる）い。
  The earth is round.

When there are no other words apart from the adjective itself, it comes across similar to ‘It’s cute’, or ‘It’s hot’, and similar expressions in English. We just have to guess what the ‘it’ is from context.
In many sentences, either は or が could be used to mark the word/words that the predicate is describing. However, which one sounds more natural will depend on many factors. In general, if you are specifically pointing out something as being different from other things in the same category, が will be used.

• このお風呂（ふろ）は熱（あつ）い。
  This bath is hot. (In general, it is hot)
• このお風呂（ふろ）が熱（あつ）い。
  This bath is hot. (Compared to other cooler baths)

In this example, が is distinguishing this specific bath as having a unique level of hotness compared to others (maybe the water system is broken and only boiling water comes out). は would sound unnatural in this type of sentence, as it is not the ‘normal’ state of the bath.