What does a bracketed sound mean in the IPA transcription of a word?

Question

I noticed that some words have a bracketed sound in their transcriptions in some dictionaries, for example, see the following from Lexico:

• locate - /lə(ʊ)ˈkeɪt/
• open - /ˈəʊp(ə)n/ (I assume here the bracketed schwa sound represents syllabic "n")

And the case of "length" is more interesting, Lexico gives two transcriptions:

• /lɛŋ(k)θ/ -- with "(k)"
• /lɛn(t)θ/ -- with "(t)"

I couldn't find any other examples. What does a sound in brackets mean in the transcription of a word?

Answer 1

"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.