What does a bracketed sound mean in the IPA transcription of a word?
"Round brackets in a transcription indicate that the symbol within the brackets is optional." Hence it may or may not be pronounced, depending on the speaker, accent, stress, etc.
This is according to the Oxford English Dictionary, whose publisher also produces the content on Lexico.
Source: "Key to pronunciation: British English", OED website, accessed 25 February 2021.