There are two main sets of numbers on your tyre.
The biggest is usually the tyre size. It can be written in three different standards, for example:
Imperial: 28x 3/4 - (Wheel diameter/inner tyre diameter x width in inches)
French: 700x 23c - (Wheel size designation x width in mm -letter is part of the wheel size designation e.g- 700c, 650b)
ERTRO: 23-622 - (Width in mm x wheel circumference/tyre, inner circumference)
The second set of numbers is typically the recommended tyre pressures. (Please see "What pressure should I pump my tyres to" for more information)