Delsus is sort of right.

When a piece of gear has an "Optimal Rank," you can wear it below that rank. However, if you're even one level below the Optimal Rank, you will not get the full benefit of the stats. (So it's not +/- 5 levels. It's just one level.)

If you're rank 25, and you equip a weapon that's got an Optimal Rank of 26, then you'll get significantly less benefit from the stats. That's true for all tools and weapons. If you equip them even one level below the Optimal Rank, your stats will be reduced by a fairly significant amount. So it's usually better to equip a lower ranked tool/weapon than one that's above you.

On the other hand, the reduction in stats for armor/gear scales. So the reduction is much less severe. You might be better off wearing gear that's got an Optimal Rank higher than you.

Also, as Delsus said, anytime you equip gear/weapons/tools that have an Optimal Rank above your job rank, your gear gets damaged a lot more quickly.

In addition, gear has certain job affinities. So some gear says that it's made for (for example) Gladiator, Pugilist, Marauder, Goldsmith, and Blacksmith. If you put on the gear while performing a different job (say, as a Conjurer or Weaver), you will get a significant reduction in stats. The amount of reduction seems to vary, but it's probably about 1/3rd.

In the future, you might consider asking these type of questions in the New Player Help forum.