Accruing an income or expense means spreading it over a fixed period instead of bookkeeping the entire amount at once. The aim of this is to make the company’s accounts more accurate, by ensuring that expenses and income that belong to each other are booked in the same period. If you e.g. pay rent for SEK 15,000 for January in December, then that rent must be accrued so that the expense is charged to January instead of December.
Or if you invoice consulting hours for the month of December in January, that income needs to be accrued to December. In the K2 accounting framework, which is used by most smaller companies, accruals are usually not done for sums less than SEK 5,000.