The poem presents a "dialog" between Spense and his lady. He attempts to immortalize her by writing her name in the sand, but the waves (time) relentlessly erase what he writes. He writes again, with the same results.
The poem them switches to his lady who tells him that he is foolish to attempt to make immortal that which is temporal; she will pass just as the writing in the sand.
Spenser then reenters the conversation and tell her that although there is a physical/temportal part to her nature, his verse will make her immortal.
He summarizes the theme in the final couplet: although things physical may perish, the spiritual nature of their love will transcend death and even grow.
When he writes her name on the sand, her name is washed away by the waves. He tries again and again but his all attempts when the tide is in will be washed. The lover here emphasize that allegorically;
The tide represents "the time" and
The sand of seashore represents his "memories"
The word "tide" refers to the word "time" also in means of written and "sand" also refers to his memories because memory is a reflection of the past and it has a particular shape in minds to indicate particular moments and events which we experienced. but this shape in time becomes uncertain as the time passed memory skips over some important or trivial details. So everthing can be forgetton, ,n memory there can be nothing everlasting JUST LİKE HER NAME ON THE SAND OF SEASHORE..
the poet has a great longing to immortalize her name. But he deduces that nature wants to explain that love is temporary. Because when he wrote her name the tide washes it away. Then the lady speaks that his effort is in vain he can not change the intrinsic nature of the mortality. She believes every mortal thing will be perished. She is offended by his attempt to immortalize her.But the lover believes when the love becomes immortal her name will be written in heaven.
The theme of sonnet 75 by Edmund Spensser is saying that no one can live forever, but he is going to immortalize her virtues and the love for each other in his poem.