First of all, which version of Solidworks are you using?
Linked dimensions used to be the way that Solidworks used to make two or three dimensions the same set and drive each other.
In other words, if you had three dimensions that were linked, then modifying one of them modified all three of them to the same number.
I don't think Solidworks uses that anymore. Now you can use global variables and use an equation to make your dimensions match the global variable.
One example would be using a simple equation in which one dimension drives the other. For example, if you want one line to always be twice the length of another one. First you'd dimension the shorter line, then, using the Smart Dimension tool you'd click on the second line. Instead of entering the value directly you'd click on the first dimension to enter it in in the dimension value box, then you'd enter *2 and click on the green check mark (or hit Enter). This would link the two dimensions so that if the first one was edited the second would update automatically.
Im on SW15 and this is the first version of SW I've used so I don't know what came before or after. I discovered linked dimensions and have had a play with them.
To link a dimension you need to create it and then right click on it and choose link values. Here you enter a name to create a new global var, or select one you have already created. This adds a little chain link before the dim. You can now change the value of this dim anywhere and eveywhere it is used it will update. It is slightly different to using equations and global variables in that you don't need to open the equations to alter it. Interestingly you can use the link variables you create as a global var. notice on the right below I have chosen to make the dimension equal to the global var created by linking using an equation (=). My best advice is to have a play with them to see how they work and when they might be useful.