Now when they drew near to Jerusalem and came to Bethphage, to the Mount of Olives, then Jesus sent two disciples, saying to them, “Go into the village in front of you, and immediately you will find a donkey tied, and a colt with her. Untie them and bring them to me. If anyone says anything to you, you shall say, ‘The Lord needs them,’ and he will send them at once.” This took place to fulfill what was spoken by the prophet, saying, “Say to the daughter of Zion, ‘Behold, your king is coming to you, humble, and mounted on a donkey, and on a colt, the foal of a beast of burden.’”
Even though Jesus wished to come into the city humbly, his followers laid palms in his pathway.
It was a practice of people's of the east to cover the ground that someone of high honor was walking on. You can see this happening in the Bible with king Jehu when the people lay their garments before him in 2 Kings 9:13. In Matthew and Mark's gospels only tree branches and clothes are described as being laid before Jesus. John is the only author who specifies "Palm" branches.
The idea is the same though: The people were honoring
Jesus the way they would honor a king. So to answer the question of why the people lay palm branches before Jesus we have to ask what the people thought of Jesus to make them honor him in this way.
So the best way to understand the Bible is to read it more and more. If the formal translations confuse you, buy a copy of "THE BOOK" published by Tyndale House (any bookstore) as it is a comprehensive Bible written in plain language that you will understand. I use it as a wonderful cross reference to bible reading as it does not take the place of the Bible but rather makes you want to read the original after reading the plain text. The Book also has many helps such as how to prayer for specific needs which is very helpful when you are in a situation and do not know how to pray for that specifically.
God bless you friends and do come again. Celebrate the coming of Christ on Palm Sunday.
My Christian heart to yours.