time axis confusion in signals and systems
Like most comments have pointed out: a causal system only has non-zero values for \$t &gt; 0\$, while non-causal systems can be non-zero at \$t &lt; 0\$. Causal means that the output can only depend on the current input and its past. Causal systems are what you'll typically find with real-time circuits, where it is impossible to look into the future.
Non-causal systems can be found typically in signal processing applications where you have access to all the data beforehand. Typical &quot;real-world&quot; examples would be processing images in X- and Y-direction, or audio processing of pre-recorded material.
You can think of it as using data left and right of the current point to determine our output value rather than only the left side or past input.