A “safety deposit box” is a box. This box is for protection. It protects the thing inside it. You can put anything in this box. People put expensive things in these boxes. People keep safety deposit boxes in a vault.
Criminals in London rob a vault. The criminals take 300 deposit boxes. That is a lot. This is a bigrobbery.
Difficult words: vault (a room which protects safety deposit boxes), rob (to steal/take things from somewhere), robbery (the situation when people rob something).