Opus is Latin for "work." This Ballade No. 1 is the 23rd catalogued piece Chopin composed. He composed four Ballades overall.
Another example for you, Chopin's two sets of Etudes, Op. 10, Nos 1-12 and Op. 25, Nos. 1 -12.
Other work, Opus 11 through 24, are other pieces composed in chronological...