A Page Layout controls the field arrangement, related lists, and buttons on a record's classic detail page. It's an object- and record-type-scoped configuration: per object you can have many page layouts, and you assign them to users by profile (and optionally per record type).
In Lightning Experience, much of what users actually see is driven by a Lightning Record Page built in the Lightning App Builder, not the page layout. The Lightning Record Page controls the overall page composition: which components appear, where the highlights panel sits, what the activity timeline shows, what tabs are present. The Page Layout is still in play — it controls the contents of the Details tab inside that Lightning page — but everything around it is the Lightning Record Page's job.
So if a user complains "I can't see this field", the answer depends on which page they mean. If they're on the Details tab, fix the page layout. If they're missing a related list or a custom component, fix the Lightning Record Page. Both have their own assignment model: page layouts assign by profile/record type; Lightning Record Pages assign by app, record type, and profile.