This is the 3rd most frequent Chinese word.
是
"To be" (am, is, are).
In this sentence, '是' is used as a verb to equate the subject with its identity or role.
我是一名老师。
I am a teacher.
Here, '是' serves as a linking verb connecting 'today' and 'Tuesday' to indicate the day of the week.
今天是星期二。
Today is Tuesday.
In this example, '是' introduces the focus or main point, stating 'how to solve it' as the essence of the issue.