Opus or the shortened form op. after the title of a piece of music means “work”. It is followed by a number. When a composer writes his or her first piece of music it could be called “opus 1”. The next composition would then be called “opus 2” etc.
Giving pieces of music opus numbers helps us to know which piece of music it is. For example: Beethoven wrote lots of piano sonatas. His Piano sonata in A flat major op.26 is a sonata he wrote when he was a young man. Many years later he wrote another piano sonata which is also in A flat major, and that one has the opus number 110.
You cannot always tell from a composer’s opus numbers the order in which the works were composed. Until around the end of the 18th century opus numbers were only given to pieces of music which were published.
Some musicologists (people who study and write about music) have studied all the works by a famous composer and have given them a catalogue number. For example: Mozart’s music does not have opus numbers. Some of them are long operas, others are tiny little pieces for the piano he might have written in a hurry one day. A man called Köchel made a list of every single work by Mozart and gave them K numbers (K for Köchel). His numbering goes up to 622. This is useful, for example, to tell the difference between his Symphony in G minor K183 and his Symphony in G minor K550.
The plural of “opus” is “opuses” in English. This is because the Latin plural is “opera” which is rather confusing.
The word "opus" can also refer to the "work" of an artist.
An artist's "magnum opus" means: his or her greatest work.