The definition of the variable <number>

< number>::=< digit > | < digit >< number > 

where < digit >  is defined as

< digit > ::= 1|2|3|4|5

Apparently reflects the following syntax diagram

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/7C6zs.jpg

Please explain why this is the case. I am particularly confused with the clause after the vertical OR line (i.e. < digit >< number >) and whether it has something to do with the fact that number can consist of many digits.