Having mental capacity means that a person is able to make their own decisions. The Mental Capacity Act says that a person is unable to make a particular decision if they cannot do one or more of the following:
It is very important to always remember that lack of capacity may not be a permanent condition. The NHS has helpful information on mental capacity here: http://www.nhs.uk/conditions/social-care-and-support-guide/pages/mental-capacity.aspx .
There is also a guide for health and social care workers on making decisions and mental capacity