I will break it down in easy parts.
"Yesterday is history": What happened in the yesterday (or in the past) is now in your past. You can't change it and it shouldn't be I'm your present thoughts.
"Tommorow is a mystery": You don't know what will happen in the future as it has not happpened yet. therefore, it is a mystery.
"Today is a gift": You are alive and hopefully well. The fact you get to live to see another day is a gift.
"that's why its called the present" Present is a term we use to describe the here and now. For example; "I am presently writing an answer on Brainly!"
I hope this helped, cheers.
