What does again in the Creed mean?
The official teaching of the Church can be found here.
He will come again to judge the living and the dead. (Apostle’s Creed)
When the Son of Man comes in his glory, and all the angels with him, he will sit on his throne in heavenly glory. All the nations will be gathered before him, and he will separate the people one from another as a shepherd separates the sheep from the goats. (Mt 25:31-32
There are two occurrences of “again” in the current translation of the Nicene Creed, and one occurrence in the current Apostles’ Creed. I will assume that you meant “and rose again on the third day” because it is the more confusing usage.
Just as you say, “I fell down and got up again”, the Creed is using a common idiom to express the fact that he rose, much like he rose in his first life every morning when he woke up. “Rose again” is a translation of the single Latin word “resurréxit”.
The translation of et resurrèxit tèrtia die: and resurrected the third day.
Crucifìxus etiam pro nobis sub Pòntio Pilàto; passus et sepùltus est, et resurrèxit tèrtia die, secundum Scriptùras, et ascèndit in caelum, sedet ad dèxteram Patris.
FWIW, the other “again” is a translation of “iterum” (which is actually, a second time) in Et iterum venturus est cum gloria judicare vivos et mortuos. (literally, “And again He shall come with glory to judge the living and the dead”)