Señal vs. signo vs. muestra
The Spanish words 'señal', 'signo', and 'muestra' all translate to 'sign' in English, however, they are used in different contexts and their meanings can vary subtly.
Señal
'Señal' is used to refer to an indicator or mark that provides information or instructions. It can be a physical signal or an intangible indication of something. Most commonly, it's used in the context of road signs, signals, or cues.
La señal de tráfico indica que debemos detenernos.
(The traffic sign indicates that we need to stop.)
No he recibido ninguna señal de él.
(I have received no signal from him.)
Signo
'Signo' generally refers to a symbol, sign, or gesture that represents something else, often used in abstract concepts or systems of representation like mathematics, languages, etc.
El signo de exclamación indica sorpresa o enfasis.
(The exclamation mark indicates surprise or emphasis.)
El "+" es un signo matemático.
('+' is a mathematical sign.)
Muestra
'Muestra' refers predominantly to a sample or specimen which is indicative of a larger group or whole. It could also mean an indication by showing or demonstrating something.
Esta es una muestra de la calidad de nuestro trabajo.
(This is a sample of the quality of our work.)
Tu comportamiento es una clara muestra de falta de interés.
(Your behavior is a clear sign of lack of interest.)
Summary
While 'señal', 'signo', and 'muestra' all broadly translate to 'sign', 'señal' is used more for tangible instructions/indicators, 'signo' often refers to symbolic representations, and 'muestra' mainly refers to representative samples or demonstrations.