Solidity allows a source file to import other source files. Here is an example to demonstrate this:
//This statement imports all global symbols from "filename" (and symbols imported there) into the current global scope. "filename" can be a absolute or relative path. It can only be a HTTP URL
import "filename";
//creates a new global symbol symbolName whose members are all the global symbols from "filename".
import * as symbolName from "filename";
//creates new global symbols alias and symbol2 which reference symbol1 and symbol2 from "filename", respectively.
import {symbol1 as alias, symbol2} from "filename";
//this is equivalent to import * as symbolName from "filename";.
import "filename" as symbolName;