I'm trying to create my first trailhead playground and it says "to go to the bottom of this page" or on any hands-on challenge and go to the Launch button, but this is not displayed for me so I'm unable to continue.

I'm trying to create my first trailhead playground and it says "to go to the bottom of this page" or on any hands-on challenge and go to the Launch button, but this is not displayed for me so I'm unable to continue.

I don't know what is the number inside the square brackets in SOQL used for?   for instance, number 0 (zero) inside square brackets. Thanks for your help.

1Account[] acctsWithContacts = [SELECT Name, (SELECT FirstName,LastName FROM Contacts)
2                               FROM Account
3                               WHERE Name = 'SFDC Computing'];
4// Get child records
5Contact[] cts = acctsWithContacts[0].Contacts;
6System.debug('Name of first associated contact: '
7             + cts[0].FirstName + ', ' + cts[0].LastName)