An employer shall bargain collectively with the representatives of its employees as defined in section 26 and is authorized to make and enter into collective bargaining agreements with such representatives. For the purposes of this section, to bargain collectively is the performance of the mutual obligation of the employer and the representative of the employees to meet at reasonable times and confer in good faith with respect to wages, hours and other terms and conditions of employment, or the negotiation of an agreement, or any question arising under an agreement, and the execution of a written contract incorporating any agreement reached if requested by either party, but such obligation does not compel either party to agree to a proposal or require the making of a concession.