there is a limited amount of frequencies. In GSM for instance, there are only 124 channels.
Each operator can receive 40 channels for instance.
In this situation, it means that 40 TRX can be activated “on air” without risks of interference. But if you want to cover a whole country, you need much more than 40 TRX’s ! Therefore you need to reuse your 40 frequencies available : several TRX’s must use identical frequencies. And in order to avoid interference, you must ensure that two TRX’s with same frequencies will not overlap their coverage, because that would generate interference.
The goal of frequency planning is to perform the best mapping of frequencies, that is, to find the best reuse pattern.