Effectively, a CSA defines the terms under which collateral is posted or transferred between swap counterparties to mitigate the credit risk arising from in the money derivative positions.
The Collateral must meet the Eligibility criteria in the agreement, which may prescribe which currencies it may be in, what types of bonds are allowed, and which haircuts are applied.
[1] There are also rules for the settlement of disputes arising over valuation of derivative positions.
Compare the "Outright transfer" offered under English Law Credit Support Annex with "Security Interest" under New York Law Credit Support Annex.
Both New York Law Credit Support Annex and an English law Credit Support Annex operate to create security interests in the collateral being posted, the differences are operational and can be material upon an insolvency of the other party.