According to the Lehigh site, a focus group provides a way to get feedback and information from a group of customers. Focus groups can be used in many types of research and can provide useful information. But it is important to conduct them in ways that make them reliable and valid.
In a focus group, a group of about 10 people meet with a moderator in a guided discussion based on a set of research questions. The emphasis is on getting detailed views of the participants, rather than getting brief answers from a larger group, as is done with structured interviews and other methods.
Reliability is the extent to which a measure (such as a focus group) is accurate and replicable. With focus groups, this could concern whether another focus group, of similar but different people, would give similar answers. Focus groups often have problems with reliability. These can be lessened if the moderator is highly trained and if questions are relatively specific.
Validity is the extent to which a measure measures what it purports to measure. For focus groups, this could mean whether it is reasonably certain that people are talking about what you think they are talking about. Focus groups tend to be strong on validity.