In simple its a directive that binds the <a> tag with a state. The format for writing it is
ui-sref=’stateName({param: value, param: value})’
now you can have the state name with or without parameters. With optional parameter the tag will look like this.
<a ui-sref = “mystate”> Click here </a>
The directive will generate a href with the associated state so this tag will become
<a ui-sref = “mystate” href=”#mystate”> Click here </a>