I don't remember ever installing an app called 'SIM Toolkit'. Can someone be stalking my phone with a SIM Toolkit?
SIM Tool Kit (STK) is a system app for managing the services provided by Service provider. The operator may provide some basic services, subscriptions other value-added services.
The STK communicate with the server using basic USSD or SMS. So, you may incur SMS cost as per your operator tariff.
Nothing to worry about it. If you are not using it you can remove it (require root access)
You can read more about STK in https://en.wikipedia.org/wiki/SIM_Application_Toolkit
Sim Toolkit is not available in play store. It's added automatically when the sim card is inserted into the phone. However, recent findings indicate that SIM toolkit causes serious vulnerabilities.