Indefinite Leave to Remain (ILR) is an immigration status which allows the person who holds it to live and work in the UK for an unlimited time, without any need to apply for a visa extension. It is also called Permanent Residency Permit.
To get an ILR status, an endorsee will have to remain in the UK for a period of 6 months per year at a minimum. Nonetheless, the rest of the year you are free to travel and can reside in another country.
After receiving ILR status you can travel freely, however, you should not be outside UK for more than two years in a row, or you may lose your ILR status.