Layoff notice, what does the 60 days mean? after 5 years as a copywriter

I received a formal notice about a mass layoff at my workplace. I do not know if I should start looking now or wait for severance news. Has anyone been through this?

I stayed for the retention bonus and looked for work on the side. It cushioned the gap.

Write down key dates and keep copies of the notice. Mine helped when a question came up later.