Databases have to support concurrent transactions. Isolation levels define the degree to which changes from one transaction are visible to another transaction (which are both running concurrently). If you do not already understand the concept behind isolation levels, please read this Wikipedia page before reading this post – https://en.wikipedia.org/wiki/Isolation_(database_systems)
Different database vendors offer a varying degree of support to the four standard isolation levels. They also differ in the way they offer these isolation levels (some may lock data while some others may not lock and may just offer a snapshot of data present at a particular time). Some vendors also offer extra isolation levels in addition to the standard ones.
This post aims to explain/compare the isolation levels offered by Oracle, MS SQL Server, MySQL and PostgreSQL.