Cheese triers are used to test the readiness of aged cheese without cutting it open. When inserted into cheese a sample core can be removed, the sample is evaluated for taste and texture, the remaining plug is then pushed back into the aging cheese.
When done testing, the opening around the plug should be sealed with a smear of the cheese sample to prevent mold from entering the cheese body. Using a cheese trier allows you to evaluate the cheese making process (flavor, moisture, acid, texture etc) earlier in the aging cycle and make adjustments needed for subsequent batches of cheese.
The time you save by sampling the cheese earlier makes the addition of a cheese trier well worth the investment.