Help me understand salary cap please!

I have no understanding of the salary cap. I know that releasing some players will take off their salary in the offseason. I also know that you cannot sign free agents unless you have the cap space (DUH)!
But there is one thing i still do not understand, i dont get how the cap works when trading! Is there a simple overview of it?

each team in a trade needs to receive players making around the same amount of money. i. e. you can't trade a player like Andre Barrett to get Kevin Garnett. Unless, of course the team receiving Garnett has enough money under the cap to pay him which is very unlikely. and as far as releasing players, it only works if another team signs them (which is unlikely) so it's best to release players in the last year of their contract to maake room for a nother player.

It's best to make a trade where the total contracts on both sides is as close to even as possible but you do have a bit of room to work with. To be precise, the total contract amounts cannot be more than 25% larger than one another. That means if you're trading contracts that equal a total of $5,000,000 the contracts you receive in return must not be larger than $6,250,000 (25% larger or 125% of $5,000,000) or smaller than $3,750,000 (25% smaller or 75% of $5,000,000).

ok, im starting to get it, i see this game requires some math

Only when adding up the salaries!(But thats what GM's job requires)