Fidelity is explained here: What to mine? Choosing between Cuckatoo31+ and Cuckaroo29
The quote is:
Fidelity of a miner is the probability of it finding 42-cycles in graphs that have them. Normally that should be over 0.999, so we can consider it equal to 1 for all intents and purposes. But when a (mean) miner is constrained by available memory, it has to spill a small fraction of edges that do not fit into the buckets and this reduces the fidelity. Notably, the cuckatoo_mean_cuda_rtx_31 miner has a reported fidelity of about 0.7