leg to hand or often known as having your horse "between the leg and hand" is a situation where your horse is pushing from behind (engagement) and you have a contact (feel) in your reins, the horse is then "connected", the ideal result of which will be for him to lift and use his back, and relax his topline "onto the bit". This overall back to front mechanism will result in your horse becoming round.
firstly your horse needs to be moving forward at the slightest touch of your leg. Your legs can be referred to as the 'propulsive aids' as when they are used they send the horse forward. Before you can even consider roundness you need to be moving forward actively (moves as soon as legs are put on) and willingly (so you don't have to put ur legs back on every half circle).
the hand refers to the connection that the reins create between your horse's mouth and your hands. This shouldnt be a pull or heavy weight but should be a light contact. As a famous classical rider (p. Karl) says, you should take the reins of a horse the same way that you would take the hands of your partner in a dance, no one is willing if they are pulled and grabbed onto the dancefloor. In the same way the horse's mouth should be treated with respect and acts as a connection between the rider and the front of the horse, rather than a mechanism for "pulling the horse's head down". If you are having trouble with the contact, a helpful tip I was told was imagine you're pushing your stomach against a brick wall- it sounds odd, but it works.
the aim of the hand (contact) is to prevent the horse "running through your hands" which is when the horse is not listening to your reins (so will probly take longer to respond to halt and half-halt, upward transitions will be rushed and downward transitions will result in them going onto their forehand). The horse will often be moving forward too fast and will holllow their back. The contact should be gentle, the horse should be carrying itself (rather than leaning on your hands), and should not be backing off the contact ( shown by the horse either throwing its head up repeatedly or tucking its nose right into iits chest).
the combination of the "go" from behind and the "listen" of the contact will connect the horse "from back to front". This will hopefully result in him using his back and becoming round, (dropping of the head/arching of neck). I say hopefully because many horses simply go "on the bit" by dropping their heads rather than using their back, but I don't think this should be on great concern to you yet. :)
hope this helps, let me know if any of it doesnt make sense