Consulting is an extremely broad subject area, but is basically engaging with experts that are giving advice on how to carry out tasks or strategies.
In technology is can also be referred to IT consulting, computer consultancy, business and technology services and IT advisory.
Basically though, it is consultancy firms advising organisations on how they can best use IT to improve their business.
Typically this is done through a contract for a period of time and a consultancy firm like Deloitte, Accenture, Capgemini, EY, KPMG, or others will be hired to help the business with a set of problems, or business initiative.