Yep it's actually mentioned somewhere in the paper: Chari et al. [10] introduced template attacks as a strong form of side-channel attacks. They first collect

Yep it's actually mentioned somewhere in the paper: Chari et al. [10] introduced template attacks as a strong form of side-channel attacks. They first collect side-channel traces from an attacker-controlled device, the so-called template. Then, they collect a single trace from an identical device processing an unknown secret. The unknown secret can then be recovered by comparing the trace to the recorded templates.