Picture a forest with barely no undergrowth but with lots of trees. Mostly normal trees, with some massive trees scattered here and there. Can one run through squares with a normal tree or do they block running in some way?
Is the normal tree significantly close to 5' diameter? If not, then they can move through them. If its that large, then they can't.
Massive trees' trunks fill a 5' square. Normal trees don't, and a character can share their space without detrimental consequences, just gaining cover as they use it in their benefit.
If the trees are large enough, as mentioned above, they block movement.
If the trees are smaller but densely packed enough to slow down movement (GM discretion, of course; it's their forest), that could constitute difficult terrain, which would allow passage but not a run. (If they're spaced out just enough, a GM would be within their rights to house rule that your progress is impeded enough to prevent running without otherwise considering it difficult terrain, but it would be a house rule.)
If they're small enough and far enough apart, though, it's just normal terrain and allows running. They might supply cover or partial cover depending on circumstance.