what do they do and how can i use them with my 404....?

You can use a midi cable to hook up to an external sequencer and sequence the samples on the 404, or use the velocity sensitivity of another sampler (MPC, ASR, software, etc.).

Back in the day when I first started out I had a 202, which had no seqencer. I midied it to my 505 ( roland Drum Machine), and made some nice bangers on it. So you could do a variety of things with it. Hope I understood your question and answered it . Peace, Duss.