At the same time; in equal degrees; two or more schemes carried on at once and driven forward with equal energy, are said to be carried on pari passu, which is Latin for equal strides or the equally measured pace of persons marching together The cooling effects of surrounding matter go on nearly pari passu with the heating - Grove: Correlation of Physical Forces, p 64
When used in venture capital term sheets, it indicates that one series of equity will have the same rights and privileges as another series of equity ('of equal step' in Latin)
Means "with equal step," equal, without preference, and in the venture capital context typically refers to ranking of your company's classes of stock on liquidation, dissolution and winding up