An aggressive, or excited attitude characterized by its loudness. The term originates from electronic communication. Sentences and words spelled with the Caps Lock button turned on --in other words, all upper-case-- is the modern written equivalent of shouting at someone. It usually signifies anger, or sometimes intense excitement.
Kelly looked at Lisa incredulously. For a moment she was speechless. "WHAT THE HELL?! HOW COME YOU CAN'T MAKE IT TO JAKE'S BIRTHDAY PARTY?!" Lisa, startled by Kelly's shouting, replied, "Why you gotta get all caps lock on me?!"