Joshua 22:5

"But take careful heed to do the commandment and the law which Moses the servant of the LORD commanded you, to love the LORD your God, to walk in all His ways, to keep His commandments, to hold fast to Him, and to serve Him with all your heart and with all your soul.”

Heart:

The heart is a muscular organ responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in the annelids, mollusks, and arthropods. The term cardiac (as in cardiology) means "related to the heart" and comes from the Greek καρδία, kardia, for "heart."
The heart of a vertebrate is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ. The average human heart beating at 72 BPM, will beat approximately 2.5 billion times during a lifetime spanning 66 years.

Soul:

The soul, according to many religious and philosophical traditions, is the self-aware essence unique to a particular living being. In these traditions the soul is thought to incorporate the inner essence of each living being, and to be the true basis for sapience, rather than the brain or any other material or natural part of the biological organism. Some religions and philosophies on the other hand believe in the soul having a material component, and some have even tried to establish the weight of the soul. Souls are usually considered to be immortal and to exist prior to incarnation.
The concept of the soul has strong links with notions of an afterlife, but opinions may vary wildly, even within a given religion, as to what may happen to the soul after the death of the body. It also shares as a PIE root of spirit.

The HEART is something that we need to have to live this life on this earth. The SOUL is linked to the spirit realm (things in the unseen).

So what does it mean for us to serve God with all our HEART and SOUL?

We do not just have actions, our motivations must be right. We do not just have thoughts or intentions, we need to put our faith to real works.