。
The water in the kettle is boiling.
Literal
Kettle [gen-の] hot-water [subj-が] is-boiling.
お湯 is specifically hot water — plain water is 水. The お here is the honorific prefix but is fixed to this word and can't be removed. 沸く is the intransitive verb for water reaching boiling point; its transitive counterpart 沸かす means 'to bring to a boil.' Note how Japanese uses が to mark the subject of a stative/perception predicate rather than は.