吧 is a mouth 口 plus the sound 巴 — a pure spoken-language character, invented late to write down a noise Chinese speakers were already making. It softens whatever comes before it: a command becomes an invitation, a statement becomes a guess.
走吧 'let's go', 好吧 'fine, okay', 是吧? 'right?' — 吧 is the social lubricant of Mandarin. Drop it and you sound like you're giving orders; add it and you're making friendly suggestions. It also spells 'bar': 酒吧 (jiǔbā) borrowed the English word, and 网吧 is an internet café.
A mouth 口 going 'bah' — the soft LET'S / OKAY? tacked onto sentences.
酒吧 'alcohol-bar' is a loanword built with 吧 purely for its sound — so a character invented for grammar moonlights as every bar sign in China.