The term tracer is used for a chemical that can be observed, i.e., has observable spectral lines, that indicates the presence of another chemical. Or perhaps more generally, something you can see that indicates the presence of something you can't see.
The term is often used regarding a means of observing distant molecular clouds, i.e., clouds of molecular hydrogen. Molecular hydrogen does not show itself very much, but generally there are trace amounts of other chemicals present, and any that emit observable spectral lines offer a way of detecting the cloud. For molecular clouds, common tracers are:
To determine the cloud size, nearby clouds are studied to determine the ratio of the tracer and the hydrogen molecules, e.g., alpha CO for CO. This is, in turn, used to estimate the size of more distant clouds where the hydrogen cannot be directly observed. The estimate's accuracy depends upon the degree to which these ratios are the same for nearby and distant clouds, i.e., the estimate depends on an assumption.