A meta description is a short blurb about the particular page of your website. This is a great place to insert keywords easily. However, you also want to include helpful information for potential site visitors to draw them into clicking on your website. This blurb will appear in search engine results pages under your H1 title tag and URL of your webpage.
Regarding Link Detox, links it diagnoses as Toxic are generally fine as they're either not indexed by Google or have malware/viruses/etc., but I recommend a manual review of any diagnosed as Suspicious. I used it recently to get started cleaning up our backlinks and some legit sites and blogs were under Suspicious simply because they didn't have many links pointing to them.
One of the reasons for a traffic drop can also be due to your site losing links. You may be seeing a direct loss of that referral traffic, but there could also be indirect effects. When your site loses inbound links, it tells Google that your site isn't as authoritative anymore, which leads to lower search rankings that in turn lead to traffic drops (because fewer people are finding your site if it's not ranked as highly and more).
Most businesses are aware that the search engines can drive massive traffic to their web properties. Not just any traffic, but traffic that actually converts into leads and sales. Even though there are other ways to gain online exposure, Google still leads the pack when it comes to helping you get the best bang for your marketing buck. This is mainly due to its huge user base and market share.
Well, yes and no. Sure, you can get hit with an algorithm change or penalty that destroys all your traffic. However, if you have good people who know what they are doing, this is not likely to happen, and if it does, it is easy (in most cases) to get your visits back. Panda and Penguin are another story, but if you get hit by those it is typically not accidental.