How Tor Browser reaches Mars Market
Tor is a network built to take a connection from your computer and send it to a destination through three relays, each of which sees only part of the picture. The directory pages all assume some familiarity with that idea. Readers who are new to it tend to find that twenty minutes of background reading makes the rest of the page set easier to follow.
Step by step, from download to storefront
Download Tor Browser from torproject.org and verify the signature on the bundle if this is the first time you install it. Open the shield icon next to the address bar and set the security level to Safest. Copy any onion address from the mirrors page. Paste it into the address bar and press return. Tor Browser sets up a private circuit, the Mars Market login page comes up, and the marketplace is yours from there.
Three relays, in plain language
When Tor Browser connects to an onion address, it wraps the connection in three layers of encryption. The first relay knows you are using Tor and forwards the connection to a second relay it does not control. The second relay forwards it to a third. The third relay reaches the onion service. No single relay sees both who you are and where you are going. That property is the structural reason a clearnet observer cannot tell what storefront a Tor user is reading, and the storefront cannot tell which clearnet network the reader is on.
What a hidden service is
An onion address belongs to a service that lives entirely inside the Tor network. It has no clearnet equivalent and no clearnet name. A reader reaches it by addressing the onion string itself, which the Tor network resolves to the service. The Mars Market storefront is a hidden service of this kind. The directory you are reading is not. The directory is a clearnet page that publishes the addresses so search engines can find it and readers can land on it without needing Tor Browser to do so.
Why mirrors matter on a hidden service
A hidden service has no name resolution, no global anycast and no built-in load balancing the way a clearnet site does. If a single onion address picks up a lot of traffic, it slows down for everyone visiting it. Running several addresses in parallel, each backed by the same storefront, is the practical way to keep a popular storefront reachable. The directory tracks those parallel addresses on behalf of readers who do not want to retype them.
Reasonable defaults for Tor Browser
Set the security slider to Safest. Leave it on the default identity until you have a reason to change it. Do not stack a VPN underneath Tor. The combination adds time without measurably adding privacy, and in some configurations it makes a Tor user stand out rather than blend in. The defaults shipped by the Tor Project are tested by people whose day job is exactly that, and overriding them is rarely a step in the right direction for an ordinary reader.