The Particle Photon and Core have built-in Wi-Fi modules, a Microcontroller, I/O Pins, Buttons, and LEDs. You will be using the Photon's buttons to configure the modes of operation of the Photon and Core.
If you hold the Photon or Core board such that the USB connector is at the top and the buttons are facing you, then the Mode/Setup Button is the one on the left, and the Reset Button is to the right. If you look closely, these buttons are also labelled right on the board, as shown in the following image:
The RGB LED is in the center of the Photon and Core, between the Mode/Setup and Reset buttons. The color of the RGB LED determines the current mode of operation. All the color codes are explained at this link: https://docs.particle.io/support/troubleshooting/troubleshooting-support/photon/.