Bert is a hypocoristic form of a number of various Germanic male given names, such as Robert, Albert, Elbert, Herbert, Hilbert, Hubert, Gilbert, Wilbert, Filbert, Fulbert, Norbert, Osbert, Roberto, Roberta, Bertram, Berthold, Bertrand, Umberto, Humbert, Humberto, Alberto, Alberta, Albertine, Albertina, Cuthbert, Delbert, Dagobert, Rimbert, Egbert, Siegbert, Gualbert, Gerbert, Lambert, Engelbert, Bertie, and Colbert.
There is a large number of Germanic names ending in -bert, second in number only to those ending in -wolf (-olf, -ulf). Most of these names are early medieval and only a comparatively small fraction remains in modern use.
The element -berht has the meaning of "bright", Old English beorht/berht, Old High German beraht/bereht, ultimately from a Common Germanic *berhtaz, from a PIE root *bhereg- "white, bright". The female hypocoristic of names containing the same element is Berta.
Modern English bright itself has the same etymology, but it has suffered metathesis at an early date, already in the Old English period, attested as early as AD 700 in the Lindisfarne Gospels. The unmetathesized form disappears after AD 1000 and Middle English from about 1200 has briht universally.