If work is in-house, people who already belong to the company or organisation do it, instead of paying an outside firm. An in-house lawyer works for that company; an in-house team builds the product themselves; in-house training is run by your own workplace. It is normal business English, not slang. Though actually it does not only mean “the workers sit in our building.” A person can work from home and still be in-house if they are on the company’s own staff. The opposite idea is sending the job out. You usually put it before a noun: in-house staff, in-house software, in-house café.