Understand of Android API works

Hi there!
I’m new in this forum. I just jump in this new word of App, i start to read the book and look some tutorial i found in internet. I know mabye this is a silly question but i have a problem how to understand how use android api, i give one example i need the use WifiManager and i found and example

WifiManager wifiManager = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);

so the first i don’t have a problem but i don’t understand why use context.getSystemService(Context.WIFI_SERVICE); and where fund this information. Please someone can help me ? where i can learn all this stuff ?

thank you soo much