That is used to introduce a restrictive relative clause.
Which is used to introduce a non-restrictive relative clause.
The garden, that is adjacent to my house is beautiful.
The garden, which is adjacent to my house is beautiful.
In the first one the use of that implies that of all the gardens around my house, the one adjacent to my house is beautiful. It is a restrictive clause because it limits the scope of the noun i.e. garden.
In the second one, the purpose of the sentence is to convey that the garden is beautiful. However, by introducing clause i.e. which is adjacent to my house, the writer intends to present additional information to describe the noun i.e. garden.