An API key is a long sequence of characters that are unique to your SolarEdge account. SolarView needs to know your API key in order to communicate with the SolarEdge server on your behalf. You obtain your API key by logging into your SolarEdge web site account and asking for a key. (You should have been provided with a login and password for your SolarEdge account by your solar installation company).
You can view an animation of the process below: