"Sealing" is done to preserve moisture inside the hair to prevent frizz and implies the use of either a silicone serum or an oil or "butter" after the hair has been conditioned and usually before applying a gel (for those who use them).
When an oil is preferred it's important to take into account that not all oils work well for sealing, especially if the hair has high porosity. In that case the best is jojoba oil because it does not penetrate the hair unlike coconut and olive oil (among others), this works well for most types of hair, but for those who want to intentionally weigh down their hair castor oil is better. Shea and other butters are good sealers too. If you go to Youtube and enter a search about sealing hair you will find a ton of videos that show how different people do it.