Clipping Mask is basically putting a graphical image inside the text.
Step 1 --> Create a new canvas layer with a width/height/resolution of your choice. Step 2 --> Choose a font and write down the text.
Step 3 --> Use an image of your choice that you'd like to have inside.
I'm using this one.
Step 4 --> Select the image and hit on Ctrl, C (copy) Step 5 --> Select the window where you typed the text and hit on Ctrl, V (paste).
This is how your layers panel should look like. (If your layers panel isn't at
the front of your screen, go to Window --> Layers.)
Step 6 --> Click on the 'Alt'-key and move your mouse between the text layer and the image layer.
Step 7 --> The cursor has now changed into the 'Clipping Mask' cursor. Click one time for the effect to take place.
Step 8 --> Change color of the text (which was black) to the color of the background image (to white in this case). Step 9 --> Click on the 'Move Tool' to change the position of the image to your choice.
You might want to add a stroke or something to make the text more visible. This is my final outcome.