Barswapping is a well known feature in ESO. Each player gets up to two bars to put their active abilities in. To swap the bars one presses the tilde key [~]. Barswapping can make you much more effecient in battle and increases your survivability.
The common way of filling the bars with active abilities is to place some damage abilities, buffing abilities and (sometimes) healing abilities in each slot in each bar. However this is not the most efficient method. The best method of filling your ability bars is by giving each bar a speciality.
For example by simply only placing damage dealing abilities in one bar, and only buffing abilities in the other. Or by placing healing abilities in one bar, and only tanking abilities in the other.
Barswapping becomes more important when you use the method of specialising your bars. Since you will not do much damage with your buffing bar. Practicing barswapping is easy: make sure you know to find the key binded to barswapping and place your abilities on the bar in an effecient way. Rebinding the barswap key (the tilde key) to an easier reachable key (or mouse button, if available) may be necessary to increase the speed in which you can barswap to your buffing bar and back. Remember: the faster you can barswap, cast your buffing abilities and barswap back, the sooner you get back to dealing damage.
Also the position of the abilities you place in your bars matters. I tend to place the longest lasting (with the longest duration) abilities in the first slots. For buffing I hit the tilde key, press 1, 2, 3, 4, 5, tilde key and am back to dealing damage. With (longest lasting) 1 > 5 (shortest lasting) most buffs will expire at approximately the same time. Of course you do not have to fill each slot in the buff bar with buffs. No, I sometimes fill in the 3rd slot with a finisher or escape ability. The same goes with your damage dealing bar. Slotting a panic ability in the easiest reachable slot is highly recommended. For example my healer has Breath of Life slotted on both bars binded under the  key. Some abilities need to be slotted on both bars to remain active (e.g. Mage Light and Summoning Creature abilities). Do not fill up an ability slot on your bar with these kind of abilities. Place them in a slot which is harder for your fingers to reach. Since you only have to cast it once in a long time.
Barswapping and slotting your abilities in an efficient way can make you as a player more efficient. It also makes your character more powerful since you can utilize your buffs to deal more damage, heal more efficient or tank for a longer period of time.