Portals are used as means for rapid travel all over the world. A person needs to be familiar with the destination when going through a Portal.
The misuse of the Portal, specifically not being able to properly visualize the destination, not having a specific place in mind, not having been to the destination in mind, or not knowing where the open Portal leads to before entering can cause the person to get stuck in limbo.
Most Portals need to be created and set up by warlocks, whom Shadowhunters employ for this purpose. Warlocks are able to easily create Portals with just a wave of their hands. A way to get around this is the set up of permanent Portals for transport by some Nephilim. The only known Shadowhunter to have successfully made a Portal without aid from a warlock is Clary Fairchild, who first managed it in 2016 with an ancient rune.
In this other dimension is a Portal—the only Portal left standing in that world. This Portal behaved differently in that the risk associated with its use is less than the ordinary portal: the Portal can be used to take the traveler wherever, or to whomever, he/she thinks of. Jocelyn Fray somehow got a hold of a shard of this Portal and gave it to Clary, who later found this Portal and, after using it to get to Valentine Morgenstern's Portal in Chernobyl, had Magnus Bane destroy the Portal.