If you are sending some light to a surface there are two ways to send more energy.
First, send more photons, each with the same energy. This increases the intensity, but keeps the frequency (and thus the energy per photon) fixed.
Second, increase the energy per photon. This requires increasing the frequency.
You could do neither (and thus not increase the energy) or either (you deliver more energy) or even both (that would be like sending more envelops and sending more money in each envelope).
For the photoelectric effect, a single electron needs to get at least a certain amount of energy to escape the metal. So when you tune the frequency down you can still send energy to the metal but not eject electrons out of it.
If the intensity was absolutely huge you might be able to get two photons to interact with the same electron and get it out, but that would be a very small number. And even if the intensity was quite low you could still get each photon to knock an electron out if the energy of each photon was high enough (which requires the frequency be high enough).
So just think of frequency as telling you the energy per photon, and intensity telling you the total energy being delivered.
For the photoelectric effect, the energy per photon needs to be high enough to eject the electrons and the total intensity will determine the total number of photons, which can scale up the number of electrons emitted (if any are).