The prime that defines the field over which BabyJubjub is defined.
Decimal value: 21888242871839275222246405745257275088548364400416034343698204186575808495617
Hex value: #x30644E72E131A029B85045B68181585D2833E84879B9709143E1F593F0000001
This prime is 254 bits long.
The fact that this prime mod 4 is 1 is relevant for EdDSA.
The fact that this prime mod 8 is 1 means there is no simple formula for modular square root. However, there are reasonably efficient algorithms that require a starting non-residue. For a fixed prime we can often get a starting non-residue from the table in https://en.wikipedia.org/wiki/Quadratic_residue. It is the case that 5 is a quadratic non-residue for this prime.