self isolation - you don't leave your house for anything (I assume advice would be given about how to manage a medical emergency)
lockdown - depends on where you are, each lockdown area/country has different rules.
You cannot pop out for a walk, visit the park, go to a communal garden, go for a drive etc if you are self isolating. The government needs to make this really clear because people don't seem to be able to grasp it. That is worrying.