It depends on context. In terms of an academic paper, it basically means a scholarly review of someone else's work or publication. For example this article by Brian Ball is a critical notice of a book by Scott Soames. It discusses the ideas in the book, expands on some of them, casts doubt on others, suggests alternatives. Critical as in literary criticism.

In terms of a product, particularly a computer, a critical notice is a warning that there's a severe problem, and normally accompanies steps to remedy the defect. Critical as in acute.

There may also be other popular uses that I'm not aware of.