What does Shakespeare refer to the ‘ever fixed mark’?

The ever fixed mark is used for light house which stands in the sea. The light house guides ship to follow the correct path. It doesn’t leave nor stops its work in spite of attack of sea waves and sea-storms. True love is compared to light house. It signifies love that is fixed or constant and does not change in adverse circumstances. Love which alters is not true love and such relationships developed with weak bonds of love are not pure and do not signify the real nature of love.