It means getting the nucleus to absorb a certain energy above its ground state. Since it is a quantum object, it can only absorb/emit energy in very specific amounts at once (“quanta”).
The details of how the nucleus manifests that extra energy are complicated, but you can imagine it as like, picking up a certain vibrational frequency.
The details of how the nucleus manifests that extra energy are complicated, but you can imagine it as like, picking up a certain vibrational frequency.