It's probably written in FOCAL, a DEC dialect of BASIC (sometimes told as "BASIC on stereoids"). The terminal looks like a vector terminal, so you basically drew line commands, as if you would plot on an oscilloscope.
From my perspective, the most stunning things about PDPs is the processor's console. Every fucking bit in this processor state has a light bulb (not even an LED). And this array of switches? That's how you can load some address/value into the CPU register. It's purely amazing that you can steer the machine on such a level but also use it for high level programming.
