225/45/R18
225 means the width of the tyre in mm. So 225mm wide.
45 means the profile, so the difference in length between the edge of the rim to the edge of the tyre. But this is worked out as a %.
45% of 225mm is the height of the tyre.
R means that the tyre is radial.
18 means the size of the alloy wheel that it would fit. 18". Won't fit anything else.