Under the poor laws of Elizabeth I and James I, the parish was responsible for maintaining people who couldn't support themselves.
This was supposed to mean that there was no excuse for begging, which otherwise was an attractive option to too many able-bodied people.
But obviously people tended to head for the parishes that were known to be more generous. So they invented the concept of settlement. The parish where you were settled was responsible.
You were settled where you were born, until you lived somewhere else for a year. If some other parish allowed you to stay for more than a year, they took you on. But during the first year, they could ship you back home or claim reimbursement from your parish of settlement.
They could still get lumbered, if they couldn't find out where you were settled or get that parish to acknowledge the fact. This made parishes wary of accepting any incomers.
The solution was the settlement certificate. Before leaving home you got a letter from your parish which agreed that you would still be their responsibility for the time being and they would take you back or cover costs.
When you rolled up in some other parish, you could show this to the local authorities and they might let you stay for a while. Otherwise, you might get moved on very quickly.