Y2K is an abbreviation for "year 2000." As that year approached, many feared that computer programs storing year values as two-digit figures (such as 99) would cause problems. Many programs written years ago (when storage limitations encouraged such information economies) are still being used. The problem was that when the two-digit space allocated for "99" rolled over to 2000, the next number was "00." Frequently, program logic assumes that the year number gets larger, not smaller - so "00" was anticipated to wreak havoc in a program that hadn't been modified to account for the millennium. This situation was sometimes referred to as "the Y2K problem" or "the millenium bug."
So pervasive was the problem in the world's legacy application payroll, billing, and other programs that a new industry sprang up dedicated to helping companies solve the problem. IBM and other major computer manufacturers, software houses, and consultants offered tools and services to address this problem.
Given the automation of modern life, some feared widespread and severe disruption to critical services as the year 1999 ticked over to the year 2000. However, very few serious problems were reported.