Harder rubber compounds suit soft terrain surfaces while softer rubber compounds suit harder surfaces.
With the varying nature of dirt bike riding means you will usually encounter an array of different terrain, so unless you have a factory race teams’ budget for tyres you will more than likely need to settle for an intermediate which will handle most things you can throw at it.
A hard terrain tire is made of a softer compound to give you more grip on hard and slick surfaces, whereas a soft terrain tire is made of harder compounds so the tire can dig in more for traction. If you don't always know what terrain greets you on riding day or you typically end up riding both hard and soft terrain, grab a set of intermediate tyres.
Have a good think about the terrain you are riding on most as with an intermediate tyre there will always be a compromise in performance in one terrain compared to another so choose wisely!