What exactly mean by Transaction per second in LoadUI

In a senario i have aTestcase and in that testcase i have 3 test steps.

here the Transaction per second in loadui indiacates the testcase? or TestStep?

 i dont understand what exatcly it mean by TPS..

in the table under the Test Step Metrics it shows 25 TPS

so this means there are 25 transcations per sec 

i ran the test for 1 min so it should be 25 *60 =  transactions = 1500

so 1500 transactions are completed ???

if completed why i dont see 1500 Assetions