What discovery means
Discovery is the formal pre-trial stage where each side must disclose relevant information and documents to the other. It can include written questions, requests for documents, and depositions. The purpose is to prevent surprises at trial by putting the relevant facts on the table in advance.
For a co-parent, the important consequence is that things you have written may be requested by the other side, including journals, messages, emails, and expense records. This cuts both ways, and it is a reason to keep your records factual from the start: write every entry as if a stranger, and the other parent's attorney, might one day read it.
Example: a journal full of calm, dated observations survives discovery well and may even help you. One laced with insults and speculation can be turned against you, because now the other side has it too.
The rules on what must be produced, and any protections that apply, vary by jurisdiction and are a matter for your attorney; the safe habit is simply to document as though your records will be seen.