Weapons and armor have materia slots. If any two or more materia share the same weapon slots or armor slots, they can be combined (see Materia Slots for more information).
For example, a one-handed weapon, a longsword, has 2 slots. You place both an Earth and Water Materia in the weapon slots. You combine the effects of both and sling a ball of mud right into the enemy’s eyes.
Water, Fire, and Earth can combine to make a Volcanic Geyser blast which deals 4d6 fire damage in a 20 foot burst, and creates shallow lava in the area which lasts for 1d4 rounds.
Or maybe you want to combine a green and yellow effect. You use a Novice Push and a Novice Force to give a bull rush action a little “kick”.
Be careful when using combinations though. Using 2 combinations uses up twice as many charges (2 water, 2 earth, for a total of 4 spent). Three combinations uses up 3 times the charges, but has extremely powerful effects. 4 or 5 materia combinations create effects that cause even combat hardened veterans to cower in fear.
It is said that a great mage once used a 6 materia combination in mortal combat with a great creature that is now locked away behind a gigantic wall of ice (Ice, Water, Shield, Force, Gravity, Earth). Such power has not been seen before, or since.