What does this poem by Yan Su mean yesterday, today and tomorrow?

Ask,

How many days are there in a person's life?

Do the math,

A person's life is only three days.

The past is yesterday,

It will arrive tomorrow,

Walking is today.

Don't forget yesterday,

Carefully plan tomorrow,

Seize today.

I hope tomorrow,

Today is yesterday,

You're still with me.

Spring dreams are traceless, and autumn nights are lingering.

Like singing the years, like running water passing by.

I hope tomorrow,

Today is yesterday,

I'm still in your heart.

The author's intention in writing this poem is: don't forget yesterday, plan tomorrow carefully and make good use of today. I hope you will still be by my side when time passes, and I will still stay in your heart to express my best wishes.