Hi Welcome to the Gmat Club,
THAT should be used to introduce a restrictive clause.
WHICH should be used to introduce a non-restrictive clause.
Changing that to which or vice versa can completely change the meaning of a sentence.
for eg 1. My car that is blue goes very fast.
2 My car, which is blue, goes very fast.
the first sentence uses that – suggesting I own more than one car (and even implying my other cars might not be so fast).
This is what happens if we leave out the clause and write: My car goes very fast.
The sentence’s meaning has changed..............
However, the sentence using which simple informs the reader that my car is blue. We can take the clause out without losing any essential information:
My car goes very fast.
Hope it helps!!