A liberal is a person who believes in individual freedoms, equality, and social justice. They support government programs to help those in need and advocate for progressive policies [policies that promote social change]. A liberal may support same-sex marriage, universal healthcare, and environmental protection.
Very Simple Definition:
A liberal is someone who wants everyone to be treated fairly and believes the government should help people who need it. They think everyone should have the same opportunities and support things like protecting the environment and giving people healthcare.