regex - What does $1 mean in Perl? -

What does $ 1 mean in Pearl? Besides, what does $ 2 mean? How many $ number variables are there?

$ number in variable

for example Take the following string:

ex> $ text = "Quick brown color over lazy dog The fox jumps. ";

after statement

  $ text = ~ m / (b. +?) /;  

$ 1 equals text " brown ".