A slot is where you can put you rune, item and skills.
Equipment slot is where you can put any Equippable Item to your character. There's a Head Slot for the characters head gear, Body Slot for Armor, Hand slot for hand gear, Feet Slot for feet's gear and Other Slots where you can put accessories for your characters.
Rune slot is where you can equip the Rune you got or buy. Characters can Equip the Rune in their Right Hand, Left Hand and Head. Average Magician can gain 3 Rune Slot. While Average Fighters can gain 2 Rune Slots. Also a well balance characters can gain 3 Rune Slots.
Skill's Slot is where you can equip the Skills that the chracters learn. In Suikoden Tactics. When your characters reach it's Max Level you can gain 8 Skills Slots. While in Suikoden V you only have 2 Skills Slots.
In Suikoden you'll know if the Slots are open if it is have a ------------- in it.