What is a bait?
A bait is when a player intentionally puts a unit to attract the opponents attention, normally for the opponent to assault, shoot, or move closer towards the bait.
If the opponent assaults the bait, there is a high chance for the opponent to wipe that squad out, which leaves them in the open for fire. In the case of my Doom Angels, I use a squad of marines as bait, then prepare a counter charge behind, this is my currently used tactics and it does cost me a lot as a squad of Assault Marines are quite pricey (Codex: Blood Angels).
Shooting a bait draws fire from the core "damage dealing" part of the army, which causes the opponent distraction as a higher threat moves in. It's like facing a brood of Tyranid Gaunts, shooting them can thin them down, but are you gonna stand and shoot or run from the advancing Genestealers?
You should do your best to ignore baits, unless you can wipe them out by a good round of shooting and still hold your ground, you're good.
Direct baits are baits that are obviously set up in front of you. Normally these baits have a low level of threat and is just a juicy target to go for.
Indirect baits are more interesting. These happens when a player brings out everything they have and they all have an extremely high threat level this will definitely confuse the opponent to think what they have to shoot first. Shooting one thing can mean that the other survives to cause a lot of damage.
Imagine this, within 12" you have a Broodlord with his Genestealers, a flying Hive Tyrant, a close-combat Carnifex somewhere behind the Carnifex a brood of leaping Tyranid Warriors that has Strength 5, Initiative 5 and they can rend. What do you shoot first?
In this case, I would ignore the Carnifex open fire at the Broodlord and his Genestealers, these things can mow down anything in close combat and the Carnifex is just to slow to do anything. Yes, the Hive Tyrant will cause damage, but if you were to calculate, a charging Hive Tyrant only has a maximum of 6 attacks, but a Broodlord alone already has 5 and not to mention, the Genestealers.
If you are a close combat army, it's best to give up a flank and reposition to slowly pick off the slower moving things in the army, it also depends on how you would want to deal with the baits.
Let me know how you use or think about the baiting system by commenting in this post, players should learn about this to ensure that they do not repetitively lose without realising they're falling into traps.