If (object['letter']) meaning

Hi,

I am reaching the end of the interviewing lessons. In the last two puzzles I had to use the following if statement

If (object[‘letter’])

I am having a hard time understanding what it means. I do understand what it does in the context of the puzzle, however most if statements I have seen before are very straightforward, they could al be written as a simple sentence in plain text, such as “if a is greater/smaller than or equal to b”

Could anyone help me to put a sentence to this if statement? Thanks very much!