The term chronometer is a big selling point within the watch community and it shouldn't be taken lightly. A very small percentage of watches made hold that label and it increases the value of a watch significantly. So what is a chronometer in a watch?
If you look at the root of the word, it simply means "time measurer" which is what a watch is. But beyond that, in order to be considered and chronometer, the movement withing the watch must be tested and adhere to very strict accuracy standards.
In a 15-day process, the Official Swiss Chronometer Testing Institute or COSC, puts each individual movement through vigorous evaluations. Only after the movement proves it meets these requirements, does it get to hold the title of "certified chronometer". The most commonly known standard, though by far not the only, is that the movement keeps time within an average of -4/+6 or 10 seconds a day.