Semantics
The meaning a visual sign or symbol generates through its context and use: shared, carried by the sign itself, as opposed to Interpretation, which is a personal reading.
In practice
Semantics is the meaning a visual sign carries through its context and use, broadly shared by its audience. A red octagon at a road junction means stop to nearly every driver, while the same red in a financial chart may mean loss. Designers think semantically when they choose a color for an error message or a shape for a button, relying on meanings people already share.
Often confused with Interpretation
Semantics is the shared meaning a sign carries for its audience; interpretation is the individual reading one person brings to it.
Try this
List the meanings red carries on a road, on a bank statement and on a holiday card, then compare them.