Consent (in BDSM) is a very important thing because it means that everyone involved in the activity agrees to do it and nobody is being forced to do anything against their will. It's important to talk about things with everyone involved before starting, so everyone knows what's expected and everyone is comfortable with it. Basically, everyone involved should agree that it's okay to do and if anyone says no, then it means no and it shouldn't happen. It's like a group of people deciding to go to the park - everyone needs to agree it's okay before they go.