Common sign in a park in the U.S.:

Please leash your dog.

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It can also mean"to restrain". The Oxford dictionary gives this example "‘his violence was barely leashed’" More examples here: https://en.oxforddictionaries.com/definition/leash.

The opposite is "to unleash".

The verb 'unleash', used in a figurative sense, is common. Dan Smith has given a good definition and examples of how we use 'unleash'.

As for 'leash', I never use this word in any form. 

If I were to take a dog for a walk, I'd put him him on a lead. I'd tie the lead to a post if I had to leave the dog somewhere for a short while.  Then I'd untie it.  I really can't think of any situation where I'd need to use 'leash', either as a noun or as a verb.

You needn't worry about it. There are far more important words for you to think about.

The noun "leash" refers to a flexible tether, often made of nylon or leather, often attached to a dog's collar to prevent it from running away from the owner. The verb "to leash" means "to keep something or someone on a leash." It is most commonly used in the past participle form, "leashed."

1) "Please keep your dog leashed."

2) "Leashed dogs are allowed in most Massachusetts state parks."

The direct form is possible, but just doesn't apply to many situations.

3) "I leash my dog when I take him for a walk."

4) "In the dog park, I don't need to leash my dog."

The form "unleash" is often used figuratively, to mean "finally allowing the full use of the power of something." It is common in advertising. It is also common in political rhetoric, suggesting that something will happen once artificial restrictions are removed.

5) "Our new smartphone finally unleashes the power of Android to provide you with everything you have always dreamed of."

6) "The combination of trade wars and a hot job market could unleash the forces of inflation."

7) "It is time to remove environmental restrictions and unleash domestic energy production."