"Discovery" is the phase or portion or period of a lawsuit during which the parties (the plaintiff(s) and defendant(s)) can obtain information from each other--and also from non-party witnesses or others with relevant information--using the legal mechanisms of "discovery," such as--
Written interrogatories (questions)
Requests or Notices to Produce Documents
Requests for Admissions
Discovery lasts typically 2 - 4 months, but the exact time depends on the specifics of the lawsuit and what is ordered by the judge in the particular case. Any information or documentation (with "documents" including emails, text messages, photographs, videos, spreadsheets, etc.) which is either itself relevant or which will lead to finding other relevant information can be obtained.