Informed consent is one of the founding principles of research ethics. Its intent is that human participants can enter research freely (voluntarily) with full information about what it means for them to take part, and that they give consent before they enter the research. Consent should be obtained before the participant enters the research (prospectively).
As researchers, we are bound by rules of ethics. For example, we usually cannot collect data from minors without parental or guardian permission. All research participants must give their permission to be part of a study and they must be given pertinent information to make an “informed” consent.