'ber, is a shortened use of Scrubber, or someone who is likely to mug you.
'Ber was adopted as a warning system through-out the town of walsall, if someone looked a threat the word "'ber" could be shouted and the group could escape.
"mobile 'ber" is the same thing except on a bike.
Andrew:Is the coast clear?
Craig: No! theres a 'Ber
everyone: *escapes, unharmed*