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Sampling refers to the portions of the benchmark program over which data is collected. By default, data is collected over the entire run of a benchmark. For many benchmarks of interest running the simulator this way would take much too long, weeks for SPECcpu2000 benchmarks (10 January 2007, 12:15:12 CST). An alternative is to run the simulation over the entire benchmark but only collect data for certain parts, those parts are called samples. If done successfully the data collected in a sampled run will be sufficiently close to the data collected in an unsampled run and the host time for the sampled run will be much less than for the unsampled run.
Normally simulations are performed to determine how long it would take the target to run the benchmark (or to answer some more specific timing question). Those would be done with trace mode off, the default. If timing information is not needed the simulator can be run with trace mode on, the simulator will run much faster.