When you come across an asset with a [+] next to it in the Markets view, you can click to reveal all the issuers offering a version of that asset. These are tokens offered by different issuers, but that are anchored to the same out-of-network asset.
Anchored tokens can be thought of as "tethers" to non-Stellar assets, like fiat currency or tokens from non-Stellar chains, and the ability to easily issue and redeem anchored tokens (aka "tethers") is one of Stellar's most powerful capabilities. Not all anchored tokens come in multiples—there is only one BAT, for instance—but many popular assets do.
There are five different BTC tokens (at the moment) because five different entities are offering to hold your BTC in return for a credit on the Stellar network.
If you buy a BTC token from one of those anchors, you can, at any point, redeem that token with the anchor for the equivalent amount of BTC on the Bitcoin network. For more on redeeming anchored tokens with issuers, check here.
You can also trade that token on StellarX, and since different anchors offer different rates for the same asset, there’s an opportunity here for experienced traders.
However: before you choose one anchored BTC token over another, make sure you trust the issuing anchor, and that you understand the conditions for that token’s redemption. Not all BTC tokens are created equal.