There is / There are

The phrases " there is" and "there are" indicate that something or a group of things exist or do not exist. e.g.

                                               a) There's a fly in my soup 

b) There are several mistakes in this report.

 

Note that it is possible to say the following: 

                                                  a) A fly is in my soup. 

                                                  b) Several mistakes are in this report.

Using  the pronoun there as a subject in these example sentences stresses the fact that the "the fly" or "the mistakes" exist. By using "there" in the sentences, the speaker makes a clear declarative statement, drawing more attention to his statement.