Between client engagements.
IT consultancies are companies who make their money by sending out their employees (programmers, DBAs, business analysts, etc.) to do software projects for their client companies, usually on-site
at the client's office.
An employee who is "On the bench" is not on-site, but is sitting back at head office waiting for billable work. They might be doing something useful like self-training or working on an internal system, or they could be just slacking off and editing Urban Dictionary.
But either way, IT consultancies make money by billing their clients for the hours that their consultants spend there, so being "on the bench" costs the company money.
Being on the bench isn't the same as being unemployed - a person on the bench will still get their monthly salary. However, it may lessen their bonus or feed into decisions for who gets laid off. Consultants understandably become worried that nobody wants their skills if left on the bench for too long, especially if other consultants are in demand.
The phrase comes from an analogy from team sports like football or basketball - some of the team are on the field playing, and the rest are sitting on the bench waiting for their turn to go out and play.
John has been on the bench since he rolled off the Megacorp billing system project last week.