postgresql – Table psql Description

in psql if I do d +, I get something like

    Scheme | Name | Type | Owner | Size | Description
-------- + ------------------------- + ---------- + ---- ---- + ------------ + -------------
Norsed Routing Result | table | morten 16 kB |
norsar Routing Result_id_seq | sequence | morten 8192 bytes | 

and so. The last field, "Description" is always blank. Where does that field read its content? – and therefore, how can I establish a description in a table? In some cases, that could be quite useful, the same for the description of the columns that are shown if I do d +.

I tried set ECHO_HIDDEN ON, but that did not make it much clearer.