Golang变量声明的语法含义[重复]

I am going through some Golang code and came through this block of code, I am not able to comprehend it. I can see that d1,d2,d3 are variables of type int. But what is the last string json:"start"

type obj1 struct {
    d1   int `json:"start"`
    d2     int `json:"end"`
    d3 int   `json:"retries"`
}

</div>

d1, d2, and d3 are not variables, they are struct fields. The string is a "struct tag".

This is all described in the Struct Types documentation in the language spec.