The default behaviour of PostgreSQL is, that all data, indices, and management information is stored in subdirectories of a single directory. But this approach is not always suitable. In some situation you may want to change the storage area of one or more tables: data grows and may blow up partition limits, you may want to use fast devices like a ssd for heavily used tables, etc. . Therefore you need a technique to become more flexible.
Tablespaces offers the possibility to push data on arbitrary directories within your file system.
CREATE TABLESPACE fast LOCATION '/ssd1/postgresql/fastTablespace';
After the tablespace is defined it can be used in DDL statements.
CREATE TABLE t1(col_1 int) TABLESPACE fast;