en_tn_lite_do_not_use/jas/03/05.md

boasts great things

Here "things" is a general word for everything about which these people are proud.

Notice also

"Think about"

how small a fire sets on fire a large forest

In order to help people understand the harm that the tongue can cause, James speaks of the harm that a small flame can cause. Alternate translation: "how a small flame can start a fire that burns many trees"