That generally means your weapons are each from a different set, and you have only 3 armor/jewelry pieces that match each set. So you might have a Deadlands Demolisher sword & board on your front bar that you use for bashing people, and have a weapon of a different set on your back bar that would buff whatever skills you have on that bar. You wouldn't get the Deadlands Demolisher 5-piece bonus on your back bar because you'd be missing the two "pieces" from your front bar weapon.

The benefit is that it frees up two body/jewelry slots for other stuff that might be more valuable. You can also include arena/trial weapons that are their own 2-piece "set" and still use a monster set for head/shoulders. The downside is that you have to strategize which sets will still work when the bonus is only active on one bar. Like your Shattered Fate set probably wouldn't work as a "back bar" set because you want the benefit of the penetration when you're actively attacking things.

So, the basis of "Front Bar" and "Back Bar" sets is that overall, there are 12 item slots available.

7 armour, 3 jewellery and 2 weapons (With 2h weapons counting as 2 pieces for any set bonus)

Swapping bar, means swapping weapons. Which allows you to swap 2 set pieces each time. This can allow you to be more flexible with gear.

The simplest would be using something like Arena weapon sets which is entirely enclosed within the weapons and would allow you to switch sets to buff skills for each set.

But you have other options. For example, you can have a 2 piece Monster Set, a mythic and 3 pieces of 2 different sets active at all times and then swap between 2 different bars to acivate each 5 piece bonus at different times allowing you to effecitively use 13 items worth of set bonuses.

Outside of this, there's Basalt-Blooded Warrior set for DK's, which literally provides you with a different effect based on which bar you're on (Main Bar grants Major Heroism while Back Bar grants 14% increased Healing and Damage Shield effectiveness)

For the most part, such things don't really matter. You can easily run with 2x 5pc sets and a 2pc Monster Set or Mythic without any difference between bars.