The first number is the width (mm) of your tyres.
Next is the tyre’s profile (%). Its calculated by dividing a tyre’s height over the rim by its width. If a tyre profile is 65, it means the tyre’s height is 65% of its width.
The next number is the diameter (inches) of the rim where the tyre mounts. For example, a tyre with the 205/65/R15 94H would fit a rim with a 15-inch diameter.