The number corresponds to the size and spool capacity of the reel. The larger the number, the bigger the reel and the more fishing line it can hold.

In addition to the number, we often see letters such as C, HG, XG, PG, S, D, DH, which refer to features of the reel such as the body type, reel speed, and spool capacity.

You can refer to the buying guide for more information.

The gear ratio or otherwise the speed, is the ratio of the handle rotation to the spool rotation of the reel. It shows how many times the spool rotates for each full turn of the handle. For example, a reel with a speed of 6.1:1 means that, in 1 full turn of the handle, the spool rotates 6.1 times.