A protocol specification in ASN.1 specifies the data structures in a source-like, human-readable way. It says nothing about how those data structures are to be represented when actually transmitted. The bodies responsible for defining ASN.1 have defined encoding rules, which specify how each data structure that you can specify in ASN.1 is to be represented as a bit pattern.
There are in fact several alternative sets of encoding rules that the committees have designed over the years. The earliest set is called the Basic Encoding Rules (BER), and a later one, which produces much more compact messages, is called the Packed Encoding Rules (PER).