What does the '-= ' + in animate ({left: ' = ' +width}) in jquery mean?

Left: '-= ' +width
The final value of the Left property is the value of left existing value minus width

For example: Left: ' 200px ' means that the left end value becomes 200
Left: ' +200px ' means the same as above, the left end value becomes 200
Left: ' +=200px ' assumes that the left     end value is the current value plus 200 and the final value is 300

-= similarly

-= should be the meaning of left=left-width

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What does the '-= ' + in animate ({left: ' = ' +width}) in jquery mean?