A resonance signature was the echo trail left by an object that was able to be picked up by sensors.
Each type of object had a unique resonance signature.
When the USS Enterprise-D was in the Devolin system searching for the USS Pegasus in 2370, Geordi La Forge discovered a subspace resonance signature coming from an asteroid. The frequency variances told him that it was from a Federation warp core. To hide the signature from the Romulans, the Enterprise considered bombarding the asteroid with verteron particles, but used ionizing radiation in its place, due to the artificial nature of verterons. (TNG: "The Pegasus")