Indefinite Leave to Remain status (ILR) grants the holder permission to stay in the UK without any time limit. Holders of ILR can live, work and study in the UK without seeking prior permission. They can access healthcare and benefits and also may qualify to sponsor certain family members to join them in the UK from overseas. Children born in the UK to those who hold ILR status would automatically be British by birth.
Notably, someone with ILR status can be deported and their ILR status revoked.
Most critically, if the ILR holder stays outside the UK for continuously 2 years or more, they would lose their ILR status (their status would lapse). If ILR status lapses, they may need to apply for a new visa to return to the UK. In contrast, British citizenship can be revoked under the most serious circumstances, but it cannot be lost through any period of absence from the UK.