It's "pliers are". This word is always used in a plural form and takes a plural form of a verb.

It's "pliers are". "Pliers" only has a plural form and takes a plural form of a verb.

The pliers are on the table.

The pliers is on the table.

Pliers consist of two parts; we cannot use the article "a" because "a" means a single thing. Do not say "a pliers". However, you can use the expressions "a pair of", "two pairs of", "three pairs of", etc. to count these objects.

How many pairs of pliers do you have?

Because "pair" is a single entity and not a plural noun, we say "a pair of pliers is" (not "a pair of pliers are").

There is a pair of pliers in the toolbox.

There are a pair of pliers in the toolbox.

Instead of using "pliers" in its ordinary sense, you can introduce this word as a term. In such a situation, use a singular form of a verb.

The word "pliers" refers to a small tool made of two crossed pieces, typically used to hold small things or cut wire.

Being plural, the "pliers" requires a determiner or pronoun for plural nouns (these, those, them, they, etc.)

These are called pliers. You can use them to bend wire.

Follow the same convention with some names of tools and instruments where two things are joined together, such as scissors, tongs, tweezers, and forceps.