PPTC Resettable Fuse
The hold current is the maximum current you can pass through, and guarantee the device will not trip over the specified temperature range.

The trip current is the value where a maximum time-to-trip is characterised. Any current over this value is guaranteed to trip the fuse, within the specified temperature range.

Any current in between is a no mans land, and it may or may not trip the device. It is highly dependent on environmental temperature, current, component variation and how long its had the current passing through. The idea is you should not rely on any behaviour of the PTC in this range, as it is not consistent.

PTC devices are just resistors designed with a high temperature-resistance coefficient. They heat up as current passes through, and the resistance increases. At some point, there is an avalanche effect where the increasing resistance causes more heat which causes more resistance. This stops when the resistance is very high, and its effectively high impedance until it cools down again.