With PostgreSQL, each Table or Index is splitted in segments of (usually) 1GB,
and each segment is splitted in pages in memory then in blocks for the
filesystem.

Those functions let you know which and how many disk block from a relation are
in the page cache of the operating system. It can provide the result as a VarBit
and can be stored in a table. Then using this table, it is possible to restore
the page cache state for each block of the relation, even in another server,
thanks to Streaming Replication.

Other functions are used to set a *POSIX_FADVISE* flag on the entire relation
(each segment). The more usefull are probably *WILLNEED* and *DONTNEED* which
push and pop blocks of each segments of a relation from page cache,
respectively.

Each functions are call with at least a table name or an index name (or oid)
as a parameter and walk each segment of the relation.