The sidewall of a tyre shows a sequence of characters being the tyre size, e.g. 165/65 R14. Decoding it makes it much easier for you to choose the right tyre when you're shopping.
165 is the cross section width measured between the tyre sides in millimetres.
65 is the aspect ratio. This is a percentage to tyre width. ”R” indicates the tyre type (R - radial, D - diagonal). The last number 14 denotes the width of a rim in inches.