bolder / boulder are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
bolder: (adjective) fearless and daring. (adjective) clear and distinct.
boulder: (noun) a large smooth mass of rock detached from its place of origin. (noun) a town in north central Colorado; Rocky Mountains resort center and university town.
bolder: b·OW·l·d·er
boulder: b·OW·l·d·er
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)