Check the "stats needed" section of that weapon. The icons might not be very intuitive, but the numbers in order from left to right represent: Strength, Dexterity, Magic, and Faith. These are the minimum stats you need to wield the weapon properly.
Unlike the other stats, you can sometimes wield the weapon without having the minimal STR stat, if you two-hand the weapon. Because you get a 1.5 STR bonus when two-handing. So if you want to wield a great axe (26 STR requirement), you can do that at 18 STR if you two-hand it. One-handed it won't work, though.
Edit: also, it's possible to use certain pieces of equipment without having the minimal stats and still benefit from their effect. For example, if you use the Adjudicator Shield only for the HP regen (and not to block), you can strap it on your back and two hand your weapon and you'll still get the HP regen without having the stats to wield it. The Talisman of Beasts, which can cast both miracles and spells, requires 18 magic and 18 faith, but it can be used for support spells without meeting any of these requirements (though any offensive magic, and possibly healing miracles, will be so weak as to not be worth it at all). For example with a ToB you can cast the Second Chance miracle, and if it breaks during, say, a boss fight, you could cast Warding in a hurry to protect yourself, or something like that, all that without having the 18/18 reqs.
Ironically, if you cast Curse Weapon with a ToB without meeting the stats requirements, the HP drain becomes absolutely trivial, making CW pretty much the most OP buff, haha.