A hybrid asset is something that is partially "separate" (belonging to that spouse individually) and partially "marital" (belonging to the marriage).
Examples
House: A house that was purchased before the marriage is pre-marital (separate property), but then the parties pay down the mortgage during the marriage using marital income. The reduction in the balance of the mortgage is marital.
Pension: A spouse works at the same job from 1990 to 2020 and earns a pension. They were married from 2000 to 2010, so they were married for one-third of the time period the pension was earned over. However, due to promotions and raises, the spouse earned more towards their pension in 2000 to 2010 then during 1990 to 2000.
Retirement Account: A 401k is started before the marriage. Those contributions are pre-marital (separate property). Then the spouse continues making contributions during the marriage (marital property). The spouse still continues making contributions after the date of separation (separate property). However, the marital property gained a lot of value in a short time due to fluctuations in the stock market, whereas the contributions after separation haven't gained much value.
