Either your employment has ended and your employer is worried you will bring an Employment Tribunal Claim against them or more likely they wish you to exit the business without the worry of you bringing a claim against them.
The reason for the proposed exit can vary, sometimes it is a redundancy or restructure situation. If you have more than two years service they have to follow a consultation process and a settlement agreement avoids the hassle for them.
Sometimes it is because the working relationship has broken down or more typically with senior executive exits it is to bring change.