Quenya is the Elvish language Tolkien modeled partly on Finnish and developed for his legendarium.
Tolkien created Quenya as a highly structured language with its own sounds, grammar, vocabulary, and historical development. Within the fictional history of Middle-earth, it was associated especially with the High Elves and the ancient realm of Valinor. It remained an important ceremonial and literary language even when Sindarin was more widely spoken in parts of Middle-earth.
Quenya appears in names, poems, inscriptions, and phrases throughout Tolkien's writings. The language is not simply a secret code: Tolkien's lifelong work on its internal history was one of the foundations for his world-building.
Sindarin is another major Elvish language, while Westron is the Common Speech used by many peoples. Adûnaic is associated with Númenor. Those languages are related to Tolkien's invented linguistic world but are not alternate names for Quenya.