The issue is that a former TNA employee went to WWE with all the info on dates of contract expiry, info the WWE would not have had otherwise.
To WWE's side, they alerted TNA and fired the guy... but the lawsuit is based on the "delay" in doing so that TNA claims there was of a few days. Around the same kind of time Ric Flair, long known to be someone WWE wanted back started acting up and basically walked out. They felt he'd been "tapped up" as someone in WWE knew his contract was ending and Flair just wanted to speed the process up.
TNA has to prove that WWE used the information to influence it's talents. The best way I can equate it is football (soccer for many of you). If a player is under contract, a club has to get permission from his current club before offering him a contract, agree a transfer fee etc. IF for example the club's captain informally talks to the player and says "We'll give you x per week if you come" that is then called "tapping up" as he is being influenced outside proper channels. If the amount is more it's likely the player under contract will be disrupted enough to want the move, causing the employer to have to either match the offer, let him go or risk an unhappy player causing problems. There are pretty stiff penalties for this kind of thing and in business it's often the same, many companies have prohibitive clauses in their contracts so you can't just walk out to the competition. The only exception is that 6 months from the end of your deal you can talk to any interested club and sign a pre-deal... but if you want to leave before then a fee is still payable.
Unlike in football, contracts are not as "out there" in wrestling, most are confidental so if Flair goes to Vince and says, "my deal is up in 3 months", flair is breaching and can be sued. Sure sheets report this kind of thing but it's rare that the actual money/length details are out there unless it's wanted out there like Show's 10 year deal.
In this case, WWE appears to have done the right thing, they blew the whistle, fired the guy and have held off on any "advantage" they may have gotten from the information by not signing any TNA guys. That cannot go on forever or WWE or TNA talent could countersue that TNA with their "frivolous" lawsuit are distorting the market or restraining their trade. It happened in football and is why a player worth 100m in transfer fees can simply walk out for nothing and get paid all that money themselves once their contract runs out.
Realistically, TNA won't win anything major out of it other than making the point that they are "legit" competition in that they have talent WWE want. WWE will have to fight it as they are publicly traded and settling or being found guilty has ramifications. But there will come a point where TNA will "settle" and it'll probably be in the form of a talent deal, as they have ironically been doing with WWE since the lawsuit started. Realistically there are going to be some talents that TNA don't want to renew that WWE will want and vice versa, so I can imagine it ending up with Anderson or RVD back in WWE and a few lower level talents headed to TNA.
