A Bluhmer is a jump in a suit which the player previously would not rebid when he had the chance. This means the suit cannot really be good. Instead, it shows that the suit is bad and the player has very good support in partner's last-bid suit.
In the May 2016 Bridge Bulletin, page 40, It's Your Call, the following hand is shown:
With the following bidding:
Clearly, if South really had a hand worth a jump rebid, he would have bid it instead of 2
, so 3
must be a Bluhmer.
The Bluhmer bid shows opener that responder has no wasted values in spades despite having bid them. This is important information for opener whose bidding of every suit indicates a singleton or void in spades.
In the April 20116 Bridge Bulletin, page 37:
With the 2/1 bidding:
1 - 1N
2 - 2S * Shows 9-11 HCP
3
- 4
!
4 is a Bluhmer showing no wasted values in partner's "known" (since pard bid the other 3 suits) short suit.
9764 
KJT 
JT6 
A94