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 You can still use this address but will re-direct you to the new location.

SNOTE to download, read and automatically implement known correction instructions

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 

See here for some example SAP OSS Notes:

1171138 SAP Note – Can not process message: maximum size exceeded
Details of 1171138 SAP OSS Note issue This is related to the size Size restriction for outbound e-mails and happens when[...]
SAP service market place for support and SAP OSS Notes
The best place to find SAP OSS note information is via transaction SNOTE as it will not only allow you to[...]
SAP OSS Note processing status information and values within SNOTE
The processing status does not get set automatically but is something you set manually yourself as you process the note.[...]
SAP SNOTE Implementation Status list and meanings
Can be implemented This indicates that the note is valid for your SAP system release/support package level and contains instructions[...]
OSS Note implementation using the SAP note assistant SNOTE
OSS note implementation has been made much easier since transaction SNOTE was made available and in most cases is as[...]
Check SAP OSS note valid for your system
If you are not familiar with SAP notes please see here for more information about SAP OSS Notes and how they are[...]
SNOTE implements OSS fixes to your SAP system
SNOTE was a lifesaver when it came to note application and it doesn't seem that long ago all SAP notes[...]
Download SAP Note using transaction SNOTE
If you want to download an OSS note onto your SAP system it is a very simple process using transaction[...]