Generation of a stream of data bits 0s and 1s at a certain bit or data rate
Multimedia signals such as video,picture, music, speech have bandwidths associated with their baseband analog source nature. 

For digital processing this bandlimited analog source is passed through a analog to digital converter (into a sequence of samples), which operates at a clock-frequency determined by the signal sampling rate, $F_s$, which is also limited by the source bandwidth due to Nyquist-Shannon sampling theorem. 

Furthermore for each sample, a number of bits are used to quantize (numerically resolve) it. The number of bits is dictated by the signal amplitude accuracy requirement. For digital video, it's mostly 8-bits per RGB channel. Or for audio it can be 16-bits per stereo L/R channel. 

Hence, the multiplication of samlping rate $F_s$ and bit-per-sample gives you the raw source bit rate $R_b$. You may prefer to apply some statistical or perceptual data compression to reduce $R_b$ to a minimum. 

Then you choose a digital modulation scheme such as FSK,PSK,GMSK,OFDM... and transmit those bits through your favorite medium. Before doing this, you may increse $R_b$ (by adding redundancies) for channel transmission error correction purposes. The result is your bit-stream (not necessarily logic signal levels of 0s and 1s though) that's traveling accross the universe...

I hope I clarified it.