Physicists detected signals at the smallest distance scale ever probed. What does that actually mean?

The BBC reports researchers have picked up signals from the smallest distances ever explored, which their science correspondent calls the 'zeptoverse'. A zeptometre is 10^-21 metres, around a millionth of the size of a proton. I'm struggling to picture what 'seeing' something at that scale even means when you can't use light. Could someone explain how particle experiments measure distances like this, and what we'd hope to find there?