CT burden means secondary volt-ampere (VA) i.e. product of secondary voltage and secondary current. Note that unlike in the case of a normal transformer, an increase in the secondary load resistance of CT (i.e. increase in ammeter coil resistance) will cause an increase in secondary burden.
This is because secondary of CT can be regarded as a current source. Therefore, any increase in secondary load resistance will increase the voltage drop across it and hence result in an increase in VA or burden of CT.
Secondary of CT can be regarded as a current source. Therefore, an increase in secondary burden (secondary VA) actually means an increase in secondary voltage. This, in turn, calls for higher secondary induced emf, higher core flux, and hence higher magnetizing current. This leads to core saturation and higher ratio and phase angle errors.
Therefore, the CT burden should be kept low. Accordingly, CT errors are specified with respect to burden.