For most people who play action role-playing games, rushing is the act of speed running the game in order to gain experience and items as quickly and optimally as possible. This is especially prevalent in games like Diablo 2, where rushing is a major way of playing the game. There are also other methods of rushing that some may not be aware of but may be important to understand.
It is also the act of using a higher level character to help one or more lower level characters gain experience and loot in a really quick run through the game. While some may think that it is not how the game is meant to be played, the practice is actually quite alright with the folks in Blizzard as it is a social activity and a good idea to get new characters going.
There is another form of rushing that Blizzard does not approve of as much, and that is the exploitation of bugs in order to skip content that was not intended to be skipped. This is discouraged, and only the form of rushing first mentioned is acceptable in terms of fair play.
Equipment is mostly irrelevant in terms of rushing since those who can't handle an enemy are meant to back off and let stronger party members handle it. But for the sake of optimization, there are some guidelines for items that can be followed to make rushing easier and more fun.
For the higher level characters who are doing most of the work, the best Diablo 2 items are the ones that enable them to kill enemies with as few hits as possible, preferably with only one hit each. That must also coincide with the skills to be used, preferably splash damage, multiple target, and/or area of effect skills that can kill multiple enemies all at once.
For the lower level characters who are tagging along, the best items to equip are those that enable them to avoid getting hit and survive if they do get hit. Of course, end game areas are the target for the best loot, and staying alive against something 10 levels or more greater than them is not that likely, which is why movement speed is the most important thing in rushing.
For both higher and lower level characters in a rush, they need as much movement speed as possible in order to move through a map very quickly and avoid getting mobbed. For those who are strong enough to kill the enemies in the current map, they should keep an eye out for enemies that may attack their weaker party members. The last thing you should do in rushing is to actually have tunnel-vision and not take care of your party mates.
The most important thing in rushing is actually knowing the game. At this point, few can ever claim that they know nothing about Diablo 2 since it is now around 15 years old, but there may be those who know more about every single area than those who haven't touched the game in a long while. Regarding that, communication is crucial to keep the players informed of its last updates. There are tons of resources available for those who are not as well-versed in Diablo 2, as well as a community that is still very much alive after all this time.