关于clipboard.d.ts的问题

问题描述：ts定义文件中的 action?(element: Element): string 是什么意思？

相关代码：

​declare namespace ClipboardJS {    
interface Options {    
/**    
     * Overwrites default command ('cut' or 'copy').    
     * @param elem Current element    
     */    
action?(elem: Element): Action;    
/**    
     * Overwrites default target input element.    
     * @param elem Current element    
     * @returns <input> element to use.    
     */    
target?(elem: Element): Element;    
/**    
     * Returns the explicit text to copy.    
     * @param elem Current element    
     * @returns Text to be copied.    
     */    
text?(elem: Element): string;    
/**    
     * For use in Bootstrap Modals or with any    
     * other library that changes the focus    
     * you'll want to set the focused element    
     * as the container value.    
     */    
container?: Element;    
}    

谢谢老师