In the second aliyah G-d says again that he is taking the Leviim instead of the first-born Jews. G-d says that the first-born Jews became His on the day that G-d killed all the first born Egyptians and protected all the first born Jews. Then Moshe, Aharon, and all the Jewish people follow the inauguration procedure described in the previous aliyah (where the Leviim shave, immerse in the mikvah, and the Jewish people place their hands on the Leviim). Then Aharon lifts each individual Levi and waves him up and down, back and forth (like a lulav is waved on Succos). Miraculously Aharon waves all 22,000 Leviim in one day!
Following the above, this aliyah continues with another detail of the service of the Leviim; and that is that they should serve from 25 years old to 50 years old. From 25 to 30 they learned the service they were to perform, and from 30 to 50 they performed it.