A "ceremony" is something like a wedding, a graduation, or a funeral. There are also lots of different religious ceremonies for different religions. Ceremonies are formal events that have a strong symbolic meaning for people.
A "traditional" ceremony is one that's done in the same way that the ceremony has been done in the past. A traditional wedding ceremony for Christians living in the United States would be one where:
- the bride wears a white dress with a veil and the groom wears a tuxedo
- the wedding takes place in a church
- the bride and groom give each other rings, and have a kiss after the minister tells them that they're married
- there's a reception (a party) after the wedding with speeches, a little dancing, and a large cake
When people use the phrase "a traditional ceremony", they're usually talking about a wedding ceremony.