In the era of interstellar travel, hyperdrives/warpdrives/jumpdrives are common for vessels wishing to traverse the galaxy quickly. This is excellent for explorers/mercenaries/military craft, or those that can afford such devices, but what of the common trader or civilian with much smaller vessels? Those that have little room for a warp drive, or those that simply cannot afford the luxury of owning one? That's where jumpgates come into play.
Jumpgates, which have actually been around since early interstellar travel, allow one not equiped with warp drives or other devices to warp from one place to another using a stablized 'portal'. Two gates are linked, and a ship crosses the threshold of the portal before being sent on its way safely. It is common for merchant convoys and civilians to use these gates to travel between major trade hubs or colonized worlds, as it offers a cheap solution for the lack of warp capability.
The gates vary in size, depending on the needs of the convoys and the planets that they visit. Military vessels tend to drop small warp gates near worlds which outposts have been established, so that supply convoys can easily travel to the outpost to resupply troops. If a larger, more permanent gate is needed, a construction team comes through the smaller one and builts a bigger one.
The downsides to using a warp gate are that they are locked to another warp gate. In other words, a warp gate can't just send you to anywhere in the galaxy. It has to lock onto another warp gate, and send you there. Not out into open space. Another downside is that confirmation is needed from the other gate in order for you to use it. Think of it as a locked door. You knock, and wait for the other person to answer and unlock it for you to enter. This may be seen as a positive by the military, as to keep out scoundrels and pirates, but its an annoyance for those that are short on time.
Jumpgates are extremely common around heavily populated worlds, such as Nyx and Sol, as well as trade hubs and colonies. Smaller ones are seen near outposts and military installations. There are even a few out in the Renegade Quadrant, though these are considered dangerous and usually a military vessel is sent ahead of another ship to scout the area out in case of an ambush.
Jumpgates have no defenses, and are relatively easy to destroy. Thus, military vessels are usually stationed nearby to defend and repair the jumpgate should it become damaged.