Soulmate is a complex concept that can range from ultra simplistic to extremely complex. There are several types of soulmates with one thing in common; they all create soul ties that connect them.
Soulmates tend to be life partners not necessarily intimate but in partnerships that can span across multiple lifetimes. Their soul connections can cause them to understand each other on multiple levels and allow them to react to each other's behavior before there is ever a reaction or action.
These connections can begin at different levels and they all have the potential to grow. They can also weaken but they do not sever completely unless they are untied through magic or intent.
These connections can occur naturally or through deliberate manipulations.