In metagenomics, OTU or Operatinal Taxonomic Unit is used to classify bacteria based on sequence similarity of the 16S marker gene. An OTU consists of a group of bacteria whose 16S marker gene shows a sequence identity of 97 percent and above. It is used to classify bacteria at the genus level.
In addition to the technical definition above, a more 'conceptual' definition is that an OTU is just a way of saying:
"This cluster of things is the same, and different from that cluster of things"
without having to assign a more 'meaningful' moniker to it (like a specific species name etc.) This is why it has the suitably vague term "operating taxonomic unit", to mean these are just arbitrary labels you assign/bins you put things in almost as a placeholder.