A peer in the SCA is a person who has received a Patent of Arms (with a few possible exceptions for some Royal Peers.) They are not necessarily enrolled in an order of peerage, but enrollment is usually the norm.
For many years, there were three polling peerage orders (the Great Orders of Peerage): Order of the Laurel, Order of Chivalry, Order of the Pelican - aka Laurels, Knights/ Masters of Arms, and Pelicans. The fourth order, the Order of Defense, was added in 2015. These four are sometimes referred to as Bestowed Peerages.
In some kingdoms, the Order of the Rose is a polling order, but is not generally considered one of the Great Orders.
A triple peer is a person who holds membership in the three of these order. For many years, that meant membership in all the extant orders - Order of the Laurel, Order of Chivalry, and Order of the Pelican A double peer, obviously enough, is one who holds membership in two of the orders. With the addition of the Order of Defense, there are now quadruple peers. Typically Royal Peerages (see below) are not counted in this particular tally.
In some Kingdoms, such as the Middle and Ansteorra, Patents of Arms do not automatically accompany the Royal Peerages. What effect this has is uncertain, since it does not actually affect the Order of Precedence. Some Kingdoms do not give out any Royal Patents, some do on a case-by-case basis.