The SVEditor indexer can also be viewed as a "compiler". The indexer parses all code in the design and creates an index of tasks, functions, modules, classes. The parser will index the code as the user edits, allowing real-time updates of the index.
While very similar, Verilog and System Verilog have some non-compatible constructs in them. It is relatively common to find System Verilog structures in Verilog files which can lead to compilation errors. Check this item to treat all Verilog (.v) files as System Verilog (.sv) files.
Check this item to have the index be updated as the user types. Un-check this item have the index only be updated when requested to by the user (Project>Build).
This is the delay between the last keypress and when SVEditor updates the index.