A more usual Swedish word would be "inhoppare" att hoppa in/ stand in for/substitute someone. It basically means that you know how to do the job, but you don't have a contract, or at least not a number of contracted days/hours. You work on a 'when they need you' kind of basis (you don't need to specifically substitute someone, but can be called in when the employer has extra workload). It is also quite similar to working as a freelancer, but, at least to me, the word implies that you are waiting for calls from just the one employer.