Pari Passu is the equal treatment of multiple parties in an agreement.
For example, if 10 people each loaned $10,000 and the startup desired to pay down the debt by $50,000, each person would receive a pay-down of $5,000 if their collective payment rights were pari passu with each other.
Pari Passu can also be used for non-economic terms as well (e.g. registration rights).