They are tags:
A field declaration may be followed by an optional string literal tag,
which becomes an attribute for all the fields in the corresponding
field declaration. The tags are made visible through a reflection
interface and take part in type identity for structs but are otherwise
ignored.
// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
microsec uint64 "field 1"
serverIP6 uint64 "field 2"
process string "field 3"
}
See this question and answer for a more detailed explanation and answer.
The back quotes are used to create raw string literals which can contain any type of character:
Raw string literals are character sequences between back quotes ``.
Within the quotes, any character is legal except back quote.
