SAP OSS notes are corrections produced by SAP to know issues and security flaws. OSS was the original name for the online service provided by SAP to report issues with your standard SAP system. You would log into OSS and describe the issue you were having. SAP would then log into your system and ascertain if it was a problem with SAP standard code or with any custom code you have implemented. If it turned out to be an issue with SAP they would create a fix in the form of an OSS note.
The OSS note includes any instructions required to fix the issue, code corrections, new tables/fields, what release it was valid for and any pre-requisites.
This meant that any solution developed could be made available to other institutions running the relevant version of SAP. It's not just fixes but other instructions related to SAP installation, development and customization. SAP notes are available via the service marketplace which used to be access via URL http://service.sap.com. You can still use this address but will re-direct you to the new location.
It's not that long ago all SAP notes needed to implemented manually including any code changes, table/fields creations etc. But now the majority of notes can be implemented via transaction SNOTE. All ABAP code instructions can be implemented via SNOTE but things like new tables or fields will still need to be created manually. This is only a minority of notes though