Web is an area effect spell. If a creature makes their saving throw, does the entire web stop affecting them? Or do they have to make a new save if they move to a new square inside the web? The web spell also describes the area as "difficult terrain", but it's unclear when that would actually affect someone. Does the that mean creatures that make their save are only hampered by difficult terrain?
Someone who is within the area, but succeeds on a saving throw, is surrounded by webs but not trapped by them — but to leave the area of effect they must cope with difficult terrain, and they again risk being trapped just as if they had failed the initial save.
Why it works this way
The clauses describing the effect of the successful/failed saving throw of the web spell description is separate from the clauses that describe the imposition of difficult terrain and what happens when someone moves through the webs. The saving throw only determines the following:
If this save succeeds, the creature is inside the web but is otherwise unaffected. If the save fails, the creature gains the grappled condition, but can break free by [making various checks].
The following clauses that describe the effects of the spell on people moving through the web is separate and doesn't depend on the saving throw at all. So this is true for anyone moving through the web's area:
The entire area of the web is considered difficult terrain. Anyone moving through the webs must make a combat maneuver check or Escape Artist check as part of their move action, with a DC equal to the spell's DC. Creatures that fail lose their movement and become grappled in the first square of webbing that they enter.