Bishops are worth more than knights except when they are penned in.
What is meant by penned in? Does that mean that they are blocked by other pieces?
I believe Brian Towers and user58697 are correct, and the author wrote/meant pen instead of pin. The dictionary tells it's a small enclosure for animals, or an abbreviation for penitentiary, which seems more appropriate. Still, it's the first time I encounter this word in a chess setting, so it's not common and the confusion is understandable.
While @Bad_Bishop provides the general (and, AFAIK, only common) use of the term pin in chess, I believe the author of your quote meant something along the lines of
Bishops are worth more than knights except when they are blocked by their own pawns.
(which matches your thoughts on the subject). Ironically, those bishops are often called bad bishops, like the white one in the diagram below:
[FEN "8/5pp1/3bp2p/1p1p4/1P1P4/2B1P2P/5PP1/8 w - - 0 1"]
One could argue that all pieces (including rooks and queens) have the same value when pinned (as in the common use of the word), but this is not entirely true; a pinned piece still prevents the opposing king from moving to certain fields, and a pinned bishop covers more squares than a pinned knight.
I dont think that the OP mis-spelt. S/he really meant penned in and not pinned in.
Penned in is a phrase meaning to be confined in an enclosure. From the MacMillan dictionary online.
same as pen
We penned the sheep in for the night.
to make it impossible for someone to get out of a place
A van was parked behind me, penning me in.
to make someone feel that they cannot escape from a situation.
The application in chess would be to situations where a Bishops mobility is blocked by other pieces, usually pawns, as in the post by @Glorfindel
When a piece is attacked but cannot legally move, because doing so would expose the player’s own king to the attack; or when a piece is attacked and can legally move out of the line of attack, but such a move would expose a more valuable piece (or an unprotected piece) to capture.
Here is an example:
[FEN "4k3/3q4/2B5/8/8/8/8/2R1K3 w - - 0 1"]
The queen is pinned to the king by the bishop as the queen cannot legally move. This would expose the king to capture.