*sigh*...
SNC-Lavalin is a big construction contractor. they had contracts in libya, 2001-2011, that were supposedly won based on bribes they gave to libyan officials (including Qaddafi). SNC has been under investigation for those bribes. the government could either go big and prosecute in criminal court, possibly bankrupting the company; or pursue a negotiated settlement - which is what SNC wanted, given that it's faster, cheaper, and no one goes to jail. the attorney general rejected the request for negotiations and started a criminal investigation.
the scandal is that the Prime Minister's Office (PMO) tried to intervene. they tried to influence/pressure the AG and get her to go with a negotiated settlement. just like in the US, the AG is supposed to be independent. if the president or PM puts pressure on the AG = obstruction of justice. SNC is based in Montreal, where Trudeau's home riding (district) is, so part of the message was "don't kill SNC because it's bad for my future election prospects". apparently SNC did a ton of lobbying immediately after Trudeau was elected... and the law that allows the government to negotiate instead of investigate was enacted by the Trudeau government, leading some to say that it was created specifically to protect SNC. lately, a bunch of ministers have been resigning to distance themselves from this developing scandal. with each resignation, it's looking worse and worse for JT.
my un-hot take: JT done effed up. it was illegal. that being said, he is being held to an unfairly high standard. if you look at the previous government (Harper), this SNC scandal is a
drop in the bucket. but because JT rode in on a high horse but didn't stick the landing 100%, he's now being dragged through the mud. might cost him the upcoming election. so for one (very bad) infraction, JT might be kicked out and replaced with something much worse - but that worse replacement isn't held to the same standards, so the bad shit they'll do will be swept under the rug just like with Harper.
analogy: after 4 horrible years of trump, someone like O'Rourke is elected. he starts to right the ship, but after 3 years it is discovered that he tried to influence his AG in an illegal way. despite it being his first scandal (vs. Trump's dozens of scandals over the same period), because he sold himself as righteous he gets crucified. in retaliation, the voting public elects another trump in the next election... and will ignore that despot's dozens of scandals. how enlightened.