Semi-staples are useful cards that are less splashable than staples. Many semi-staple cards are included in Side Decks due to their overall flexibility, though they may lack synergy with most Main Deck cards.
An example of semi-staples are generic Counter Trap Cards like "Dark Bribe"/"Recall", as they can stop a wide variety of cards and aren't restrictive, unlike other Counter Trap Cards like "Armor Break" which only counters Equip Cards or "Earthbound Wave" which requires you to control an "Earthbound Immortal" to use. This makes "Dark Bribe"/"Recall" easily Side Deck-able.
Generic Extra Deck monsters also tend to be this, giving power to many Decks but may not be suited for all Decks due to their Summoning conditions. Synchro Decks, for example, favour Main Deck monsters with lower Levels, while Xyz Decks favour several monsters with the same Level. Even Link Decks, whose Summoning requirements can be as generic as "2 monsters", tend to have Summoning requirements that include a specific Type, Attribute, or a monster within a specific archetype, or that disallow Tokens after the "Firewall Dragon" controversy.