Realtime means processing of at least some of the events and tasks in your application must be completed before a set deadline. For example, coin acceptor that detects what kind of coin you dropped in has x milliseconds to figure out if the coin is to be accepted or not. If it can't do it within the time you get unhappy customer.

Realtime systems don't have to be very performant, but they don't have something like 99th percentile latency shooting up. All events must be processed within deadline.

Low latency systems are soft realtime PLUS performance.

In a low latency trading system you have to respond to market within a deadline to have a chance to do something useful but if you don't normally it doesn't mean lives are lost, metal is bent, etc.