Gears are ratios, Ratios are math.

a number, the tooth count on the front, divided by the back count.
a ratio proportional to the number of teeth on the front,
turning the one in back a number of times . how many determined by how many teeth on the gear-cog on the back..

simple example a 48 tooth turns a 24 tooth twice for one rotation of the larger
2:1 ratio 48:12 is 4:1


a 24 tooth front gear turns a 34 tooth rear gear less than once,
an 0.7:1 ratio .. that is a nice low gear

race bike high gear a 53:12 is 4.416 rotations of the back wheel for one crank rotation ..