This is the 113th most frequent Russian word.
которое
"Которое" means "which" or "that" (relative pronoun).
Here, 'которое' is a relative pronoun referring to 'tree' ('дерево'), connecting the main clause to the relative clause.
Это дерево, которое растёт в моём саду.
This is the tree which grows in my garden.
'которое' here introduces a relative clause and refers to 'платье' (dress) to provide additional information.
Я выбрал платье, которое идеально подходит для вечера.
I chose the dress that is perfect for the evening.
In this sentence, 'которое' refers to 'отчёт' (report) to convey the reaction it caused.