A rarefaction curve is used to present relationship between number of OTUs and number of sequences. It can show species richness from sampling results.
If the curve reaches plateau, it means that with more sequences, the gain of newly discovered OTUs is limited. The sequencing depth may be sufficient.
If the curve still increases fast, it means the sequence depth are not enough and more species will be discovered with more sequencing efforts.
More details on rarefaction can be found here.