What's the difference between simple past and present prefect? I quite have a doubt with the below statements. Do they convey the same meaning?

Past perfect is not used in single simple sentences. Since it refers intrinsically to a second past time, prior to some other past event, a clause using the past perfect construction must have some other close clause or sentence to refer to.

Consequently,

• Judy had visited Thailand in 2014

all by itself, is a very strange sentence. Why did the speaker use past perfect? What else happened later?

The past perfect pops up in narratives to place events in sequence:

• Judy had visited Thailand in 2014, but when she returned last year she found a lot of changes.

But it doesn't occur by itself.