Collusion is "a secret agreement or cooperation especially for an illegal or deceitful purpose." Generally, in traditional corruption circles (ie the corporate world), the word "collusion" describes when two businesses or people whose interests appear conflicting enter a secret partnership to defraud a third party. In less traumatic eras, it mostly gets used to describe businesses secretly screwing each other over, and...like..."who cares," you know?
Enter 2017: The year we were forced to confront that our own government may be involved in a wingnut-y conspiracy with Russia. Now, the word "collusion" is a part of our daily lexicon, sneaking its way into conversation regularly as we follow the shenanigans Trump got into this time.
Unfortunately, collusion is not technically a crime in the federal code. That means that, as bad as it may be that our executive branch is allegedly in bed with our enemies, nobody will actually get arrested for it. (You may remember that our friend Georgie P-Dog Papadopoulos got charged with lying to the Feds, but not with actually colluding with Russia, which he definitely did.)
Collusion is more smoke than fire in the legal sense. The Russian government, without a doubt, attempted to influence the 2016 presidential campaign. What matters from a legal standpoint as we look at the Trump campaign, however, is whether any of the Trumpeters participated in a conspiracy and/or aided or actively concealed a crime. It's very likely that if they colluded with the Russian government, they did one of these other things in the process. We just have to find the proof.
Example: The meeting between Donald Trump Jr, Jared Kushner, Paul Manafort, and the Russian Lawyer in June 2016 implies that there was an interest within the campaign in receiving Russian assistance. It's not yet clear if this meeting was an intentional solicitation of assistance. Either is collusion, but an intentional solicitation of foreign assistance is illegal. A bunch of idiots stupidly accepting the help of a random Russian lawyer without any agenda is just bad planning.