In game, Equipment specifically refers to an item you can “use” in some way.
Equipment is distinct from Wardrobe.
To use your selected item, you generally will tap or touch its icon at the top-left corner of the screen. Most items then need to have a “target” chosen to act upon; you'll touch or tap this next, while the icon is glowing. Some items act immediately.
If an item can be used only a limited number of times, or has a certain amount of charge to it, a counter or “gauge” will appear beside its icon.
See also Game Controls