In the context of the security of the RSA cryptosystem, one often hears the term bit length of the RSA key. The bit length of the RSA key is calculated using the variable of the RSA modulus N. The modulus N together with the exponent e provides the public key.
The bit length is calculated here from the minimum number of bits (numbers which can only be either 1 or 0) which are necessary to represent the RSA modulus N.
An RSA key of 1024 bits is viewed today as being secure. If one calculates in decimal places, then the RSA modulus N has 309 decimal places here.