Plaintiffs have the right to get "discovery" to prove their claims because the defendant is usually the only person/company with evidence of the defendant's wrongful acts.
A fishing expedition is where the plaintiff cannot articulate any particular type of evidence sought, or how such evidence would be useful/necessary to its case, or how such evidence would result in evidence it could actually use.
Example: P sues D for patent infringement, demands in discovery D's head researcher's notes. Not fishing. Example 2: Same, except that P demands the CEO's call logs. Fishing, b/c not reasonably related to the patent infringement claim. Example 3: Same as Ex 2, except that P alleges that the CEO knew about the patent b/c he spoke with a former employee of P before launching D's infringing product. NOT fishing, b/c now it's shown to be relevant.
How to tell the difference: If you can't articulate something relevant to your claims that the discovery could reveal, it's a fishing expedition.