The universal translator is the technology that lets Starfleet crews speak with alien species in real time, refined from an unreliable 22nd-century prototype into a near-invisible part of daily life.
In the 2150s the universal translator was still an experimental tool, and communications officer Hoshi Sato often had to build its language matrices herself or translate by hand. Her talent was one of the reasons Enterprise was able to make so many first contacts.
By the 23rd century translators were built into communicators and ship systems and worked well enough that crews rarely noticed them. Kirk even used one to speak with the Companion, a non-corporeal being. Breakdowns of the translator, or languages it could not parse, have still produced some of Star Trek's most interesting encounters.