Securities that have equal entitlements to a payment or other benefit are said to rank pari passau.
The term is most often used when new shares are issued though a corporate action, to make it clear that they are in no way different from shares that were issued earlier — and are therefore fungible.
Securities that have different entitlements are not pari passau: ordinary shares and non-voting shares are not pari passau, and neither are senior and junior debt.
The term pari passau is also used in bankruptcy and insolvent liquidation to mean creditors who rank equally for payment: which usually means that if there are insufficient funds to pay that rank of creditors they are paid pro rata.
The term also has other meanings, particular in law.