官方API解析：
Actually starts the internals of the node (spins up threads, starts the network polling and xmlrpc loops, connects to internal subscriptions like /clock, starts internal service servers, etc.).
Usually unnecessary to call manually, as it is automatically called by the creation of the first NodeHandle if the node has not already been started. If you would like to prevent the automatic shutdown caused by the last NodeHandle going out of scope, call this before any NodeHandle has been created (e.g. immediately after init())
一般不需要在自己的代码中显式调用，但是若想在创建任何NodeHandle实例之前启动ROS相关的线程，网络等，可以显式调用该函数。
