This is a textbook example of cap manipulation.
The salary of the players under the Long Term Injury Reserves (LTIR) doesn't count towards the salary cap for their team. The player is still payed, but it just doesn't count against the cap. This is to allow teams to replace a player while they're out with long term injuries. But once a player comes back the salary they earn from that point will be counted towards the cap.
But the big caveat about the cap in the NHL is that, it's not in effect come playoff time! In the postseason you can go over the cap. This is the important part.
So the Lightning put Nikita Kucherov, arguably their best skater, on LTIR this season as he underwent hip surgery, used the cap the move freed up to sign and trade other pieces to help them during the regular season. Once the playoffs started, and cap wasn't an issue any more Kucherov was started playing and ended up scoring more points than any other skater.
It was clear that Kucherov was healthy enough to return during the season, but the Bolts made a tactical move where they sat him out and only brought him out for the playoffs to maximize their odds of winning.
Some teams and players (notably Dougie Hamilton) didn't enjoy the move as they saw it as cheating the cap rules. However, the move had been done before as the Chicago Blackhawks had done the same thing in 2015. And in a bit of twist, the Tampa Bay Lightning were the only team at the time that requested a change to the rule so teams like Chicago wouldn't be able to repeat such manipulation.